Only IE and Firefox have internet access

Hi,

My machine at home is running Windows XP, still.

About a week ago, my copy of Chrome stopped being able to access the internet. IE and Firefox still could so I didn't worry much about it. Then a couple of days later I realised that Sygate Firewall, that's been chugging along happily in the background for five years, was no longer running and would not start.

That worried me, so I went to tucows and tried to download it and re-install - only to discover their built-in download manager won't download it. Same goes for download.com.

As it turns out, it seems that most programs I've got won't contact the internet. iTunes allowed me to download an update to itself, but then won't connect to the iTunes store nor the Apple download server to update the OS on my iPad.

My first thought was malware. I've got AVG antivirus installed (and some other anti-malware programs the name of which escape me) which - like everything else - can't connect to update themselves but which last updated 1st Feb so are fairly recent. A full AVG scan reveals nothing out of the ordinary. I haven't tried running any other scans.

I'm not aware that I've made any drastic changes to the PC during this time, although my kids do use it without supervision but seeing as the eldest is six, I'd be surprised if she could manage to break anything so spectacularly or in such a specific fashion.

I've flushed the DNS cache to no avail. I've also tried taking out random processes in the task manager to see if there's something blocking the network but that's done nothing either.

So to summarize:
* Only IE and Firefox can connect to the internet: most other programs appear to be blocked.
* IE and Firefox do not display any unusual behaviours
* Sygate personal firewall won't even start

Any ideas? It's pretty scary - and annoying - and I have no idea what to do next.

Cheers,
Matt