Have you had a look in Task Manager to see if anything is using up a lot of the CPU.
If nothing running System idle process should be showing about 97% and that is normal.
Also try Device manager to see if there are any yellow exclamation marks on it.
A lot of rootkits going about and normal security programs do not find them.
Kaspersky has a free rootkit scanner but I cannot remember what you call it.
Maybe give C Cleaner a run if you have not already did it.
And of course there is a defrag and chkdsk to try.
Maybe try running your security programs in safe mode.