All of the sudden my PC is running very slow, to the point that sites time out. I am running XP, I think it's professional edition. I have tried clearing my cache, system restore, running malware bytes, CCleaner, and defrag, but nothing seems to help. Does anyone have any advice?

My PC is on a network and all other computers on the network run as smooth as silk except for this particular one.

You are probably being hindered by spyware on your computer. My advice would be to download, install and run SupertAntiSpyware (the free version), with the advance permission of your computer administrator. A more aggressive cure might be to download and run DrWebCureIt, also the free version. The symptoms certainly indicate that there is "hidden background activity" that is taking precedence over what you are wanting the computer to do -- very typical of trojan viruses.