What antivirus program are you using? I found Norton was a big system hog - personally using AVG Anti Virus (
http://www.grisoft.com) which is free and Ive been very happy with. Also for searching for and removing spyware I recommend a program called Spybot Search and Destroy.
It could be slow for other reasons - there could be programs running in the background that arent needed. If you hit CTRL+ALT+DEL and look at the processes there it will give you an idea of whats running.