Encrypted wireless router?

[#1]

Much is written and said about the necessity to encrypt your wireless connection...
Some months ago, my eldest son called round, and announced that as he pulled up he easily found that my wireless router wasn't encrypted using his mobile phone, I wasn't surprised about this as it was one of those things that was always meant to do, but never quite got around to doing.
Following his visit I found the manual for my router (D-Link G604T) And carefully set up encryption at the highest level, using a 16 digit alphanumeric password for all three computers on my home network, and assumed all was Ok.
No 1 son called again today, and found that my encryption is not working. He proceeded to set it up, and found whatever he did it failed to activate.
Can you internet experts out there, in words of one syllable for someone not that knowledgeable in this area;
a) hazard a guess what the reason might be?
b) provide an idiot guide to doing the job correctly.
Many thanks.   

[#2]

What encryption did you select: WEP (64bit or 128bit), WPA-PSK or WPA-PSK2 or something else? Note you should not use WEP as it can be broken in a matter of seconds although if any one PC does not support the others you will have to.
What OS are you running on your PCs as WPA-PSK has problems with some routers with Vista.

[#3]

Peter, thanks I almost guessed you would come to the rescue. However, my ever helpful son seems to have changed the access password to my router setup!! and predictably I can't contact him at this time, I'll get back to you asap.
In the meantime 2 PC's are running Vista Home Premium, and 1 with Vista Home Basic.
As I recall from what he was doing, in the first instance WEP was not selected, but he DID select it, prior to this I "think" it was 128bit WPA-PSK2 but not too sure about this until I can get a looksee at the setup pages. My son was only looking at 1 PC, would what he was doing have been migrated on to the other 2 as well, or should they remain in the original setup? If so then I should be able to tell you what the previous setup was...when the password issue is resolved.
In all his attempts to resolve this earlier today, after "activate" a pop-up message confirmed that activation was completed correctly, but actually made no difference to the transparency. 
Thanks again Peter.

[#4]

If your wayward son only touched one PC then the other two will not work, as you have to put the pass string in all machines that access the router, you would not need to go into the router set up or need the router password (Normally admin) to do that 

[#5]

Right; I have access to my router setup on my desktop PC (not touched by my son!!) the security level is shown as "none" with the options for WEP or WPA Which is where I have left it pending advice.
This is not how I left it some months ago. Is it possible? the "upgrade" to Vista SP1 (now completed on all 3 machines) cleared my original settings? Or is this one of the Vista issues that Peter was alluding to maybe?
edit: I suppose that my router which is quite elderly now and pre-dates Vista by some margin, may be due for replacement, however in all other areas it works just fine, I'm loathe to throw money at this problem, and then find the problem doesn't change.

[#6]

getting into the router will be easy if you don't mind having to set it up to plus.net again.  look at the back, or bottom and there will be a reset hole you poke with something gentle and hold for a few (roughly 10) seconds to clear the settings.  you will be back to factory default and can then issue your own admin name and password.
there is, or was, a wizard for getting security on all machines.  it entailed setting it up on one, saving to usb stick and invoking on all other machines you want to network.  i can't recall if the router is involved, but i think not, except for setting up the security key.
wait for others to expand on what is faulty recall.

[#7]

OK... time to start doing things in stages. It is possible not all PCs will support all encryption methods, especially the WPA-PSK or WPA-PSK2 so you need to find a common setting that all PCs are happy with. The ideal is to get to WPA-PSK2 but as I said earlier, Vista can have a problem working with WPA-PSK/PSK2.
First, set no security on wireless in the router and configure each PC to also have no security and check all 3 PCs can connect wirelessly. This will prove the basic wireless connectivity.
Second, set the router to WEP 128 and select a 13 character code for the encryption. Then set-up each PC to use WEP 128 and enter the same 13 character code. Only when all 3 PCs can connect should you go to the next stage.
Third, select WPA-PSK on the router and enter a passphrase. Do the same for each PC using the same passphrase. Again only when all 3 PCs work do you go to the final step.
Finally repeat selecting WPA-PSK2 and associated passphrase and set each PC to WPA-PSK2 and the passphrase and try to connect.

[#8]

Thanks Peter. For "going out for the rest of the day" reasons, I can't get involved with this yet today. I'll get back to you tomorrow probably.
You don't mention what to do at "try to connect" if it doesn't!!

[#9]

Depends on what point that is. If its at the WEP stage you have a problem somewhere as all systems should support WEP 128. If its a WPA-PSK stage and it works on one or two but not the others than it could be they do not support that encryption and you need to use WEP 128. If you can't get WPA-PSK2 working but it works fine on WPA-PSK then use that as not all systems will support the latest encryption method.