if it is consistantly running slow, use task manager to see if you're low on memory for what you're running. it will also show what is sucking up the memory. also, fragmented drives can slow down your system.
There are also little free utilities about, (one I remember and have used is called Memturbo), that periodically reclaim unused RAM. It sounds to me though, that you may have picked up some kind of virus. Look in Task Manager and check out processes,see if any of the items in there are using up lots of RAM, then puch the name of the offending listing into Google to find if it's a virus.