Internet Problems and Declining HD Space

I've had nothing but problems since I upgraded my Dell E920 to Vista.  I updated my McAfee when I upgraded and after its normal weekly scan, SecurityCenter became unstable.  I reinstalled it from the website and now it works fine.  Then when I tried going on the internet, IE7 would freeze.  Firefox would get stuck trying to load pages.  I disabled McAfee but it didn't help.  My computer shows the connection as working and I'm still able to access Yahoo! Messenger and mIRC.

 

When I noticed my hard drive space slowly declining, I ran QuickClean, Disk Cleanup and cleaned all but the most recent System Restore points.  It came to my attention the next day that the space is STILL declining.  I have a alot of room, so running out of free space isn't an issue.

 

Any help would be greatly appreciated.