Pls Help - stupid (but *very* frustrating) PC Hardware issue....

[#1]

Summary - when I have high network traffic throughput (via built-in 100Mbps Wired Ethernet port), my keyboard response becomes intermittent..... (misses keystrokes; double characters; completely no response; etc.). When no network traffic (as seen via a Networx graph) keyboard response is 99% (only odd character missed) - but almost completely non-responsive when, for example, doing a download at 5.8Mbps. When I notice a character has been dropped, a glance at the Networx meter almost always shows an odd spike of Network traffic has just occured...
Normally using a Logitech wireless keyboard with the TX/RX using the PS/2 port - and have already changed the batteries....  And anyway, the symptom remains when I plug in and use a USB wired keyboard.
It's happened twice (or more?) before (over the last couple of years?), and has cleared whilst being "investigated". *Really* annoying when it is happening.....  Gues it might have lasted for a week or more each time...  Most of the time, keyboard function is fine - then it has one of it's "turns".....  No pattern (that I can recall) to anything that causes it, or what happens to clear it.
This time, also booted into an alternative OS on a different partition (albeit still Windows XP) - same issue, so guess it must be hardware, not software.
It's a real pain when I'm trying to use VNC to remotely take-over another PC - the VNC network traffic stops me typing....!!!
I'm not a PC hardware engineer, but I'm guessing that somehow, the hardware interrupt for the keyboard function is being superseded by something else...?? (e.g  does anything 'share' keyboard I/O ?). 
Anyone have any ideas what might be going on, or anything I can check/change?
Thanks,
P.

[#2]

Does this help?
http://www.helpwithpcs.com/upgrading/change-irq-settings.htm

[#3]

If you used another copy of Windows that doesn't rule out Windows completely.
Download and burn a copy of the Ubuntu distribution CD, and boot from that - you can run it from the CD without installing it on the hard drive.  That should prove that the hardware is/is not the faulty item.

[#4]

Have you tried using a different (updated) network card driver?
Chances are both of your windows xp installs are using the same driver.

[#5]

How often does technology make liars of us....???!!!!
I have another alternative boot on the same machine - Ubuntu installed via Windows (Wubi), so tried that - and same issue.
However, to re-test something checked before, I then plugged in a USB keyboard, and (of course - contrary to what I previously said) it worked o.k. this time.
So booted back into Windows, and tested with USB keyboard (plugged into a spare unused USB port) - all o.k. still.
Tested the batteries that I took out of the Wireless keyboard (when I assumed that they were dead) - and they were fine - so put those back into the keyboard - and it all works o.k.
The only other thing I did notice was that during the boot into Windows (after the quick visit to Ubuntu), it (re-)"discovered" a bluetooth adapter that has been attached to the PC for ages - as if it was the 1st time it had seen it ("New hardware found"....). I'm wondering therefore whether the Bluetooth adapter might be failing, and it's radio is interferring with the Wireless keyboard?  It's quite an old Logitech Wireless keyboard, so assume it works on 27MHz., whereas I understood BT works on 2.4GHz.... - but anyone know if this could possibly be the root cause....?
Anyway, thanks for the replies - hopefully it won't go again for another good few months, as seems to be the pattern.....  and if swapping batteries and disconnecting the Bluetooth adapter doesn't fix it next time, I'll be back...!!
Thanks,
P.