Strange problem - XP Pro & networking

[#1]

This is certainly odd.... I take it your firewall isn't deliberately blocking access to the router?

[#2]

I've since found that Teamspeak connects to my Teamspeak server on the internet & I've been able to talk to a friend on it.  Using netstat, most connections are showing FIN_WAIT_1, where others (mainly messaging clients) are showing ESTABLISHED, but are not able to log in.
I'm beginning to think a reinstall is the only option but I really don't want to go down that route.

[#3]

Sorted 
I finally got around to installing Wireshark on the PC.  After switching off all other network-aware programs I started Wireshark & used IE7 to try to get to my router's web interface.  After it gave up I stopped Wireshark & found a TCP checksum error amongst all the data, it also suggested the culprit might be a setting on my network interface called checksum offload.  After disabling this everything burst into life!!
Having said that, I can't get on to the www.avast.com website to renew my AV license, any chance someone can try this site?

[#4]

Glad it's sorted, Avast site is working fine here though.

[#5]

Well it looks like I'm still not out of the woods then 
This is what I get when trying www.avast.com -
Connection Interrupted
The document contains no data.
The network link was interrupted while negotiating a connection. Please try again.
This is stupid, I can't even get to it on my laptop

[#6]

What does a tracert give you for avast?

[#7]

Tracing route to www.avast.com [74.86.245.119]
over a maximum of 30 hops:
  1    <1 ms    <1 ms    <1 ms  gateway.local [192.168.2.1]
  2    26 ms    38 ms    23 ms  lo0-plusnet.pte-ag1.plus.net [195.166.128.64]
  3    18 ms    18 ms    17 ms  ge0-0-0-403.pte-gw2.plus.net [84.92.4.2]
  4    17 ms    18 ms    17 ms  80.239.193.141
  5    17 ms    16 ms    19 ms  cw-ic-122209-ldn-b1.telia.net [213.248.74.214]
  6    19 ms    19 ms    19 ms  bcs1-so-2-1-0.Londonlnx.savvis.net [204.70.193.1
18]
  7    20 ms    17 ms    16 ms  cr1-pos-0-0-3-0.londonuk1.savvis.net [204.70.192
.5]
  8  147 ms  147 ms  154 ms  cr1-tengig0-7-0-0.dallas.savvis.net [204.70.196.
213]
  9  143 ms  142 ms  145 ms  pr2-ge-4-0-0.DallasEquinix.savvis.net [204.70.20
0.169]
10  147 ms  147 ms  147 ms  er1-te-3-1.dallasequinix.savvis.net [204.70.204.
149]
11  136 ms  129 ms  133 ms  te1-1.cer02.dal01.dallas-datacenter.com [208.175
.175.10]
12  132 ms  134 ms  131 ms  po2.dar01.dal01.dallas-datacenter.com [66.228.11
8.205]
13  136 ms  134 ms  134 ms  po1.fcr03.dal01.dallas-datacenter.com [66.228.11
8.186]
14  133 ms  133 ms  132 ms  www.avast.com [74.86.245.119]
Trace complete.

[#8]

The IP address I have for Avast is different, what do you get if you try http://75.125.246.130?
If that is OK you could try flushing your DNS cache by using ipconfig /flushdns and try it again

[#9]

Thanks for that, I was able to get on to the site with that address & re-register Avast.