having reread your post it seems to me that you might have programs running in the back ground.
you might want to check your task manager and see whats running.your rig should not be running so slow.
there are a lot of good online scanners that you could use.
click on the green button on the left http://www.majorgeeks.com/O&O_Defrag_2000_Freeware_Edition_d4545.html
heres another one for you to try http://security.symantec.com/sscv6/default.asp?langid=ie&venid=sym
both of these are easy and wont do damage to your rig .i would try these be for you look any were else.
i dont think from what you said that its a hard ware problem