Computer slowdown can't work out why

Sometimes everything goes slow - typing in my web browser goes laggy, slower to respond to clicks, can't play games at all cos they run so slow. CPU hardly being used, not much RAM being used, everything is fine again if I restart.

Any ideas how I could go about working out what's causing that?

Thanks

Try following the MS instructions here, see if that helps identify the problem: How to perform a clean boot to troubleshoot a problem in Windows Vista or Windows 7

Turning off Startup freeloaders only long enough to see if they interfere ignores that almost none of them except AV need to start with WIn7, sap resources and spy on you to make extra money.

I would establish a more resilient Clean Boot from Troubleshooting Steps for Windows 7, then work through the other steps until it is resolved.