Multiple Apps Not Responding..

I've recently built a new PC, and this is the first time I've gone for vista. Was previously an XP32 / Ubuntu fan so this is new to me.

I have a rather strange issue that I can't pinpoint the cause on.

After some time (maybe a few hours), firefox will stop working. It doesn't crash, but it just won't load any pages, it just "hangs". The same thing will happen for IE. If i close firefox the window closes, but its still in task manager.

I can't kill the process, well I can click kill, but it doesnt go. I can't load up firefox cos its already running (apparently), and I can't shut the system down, presumably because of this firefox process still running.

I get the same with uTorrent, after about an hour it stops responding and prevents me from shutting down the system.

At the moment I have to just switch the PC off at the mains, and I'm not sure this is a healthy practice.

I was running Avast, Zonealarm. but I have since uninstalled those. I still have the same issue, even if i disable the windows firewall etc.

Any ideas? I've tried googling but it just returns threads suggesting to uninstall antivirus/firewalls