re: PC or MAC?

Download (free), install and run these:

CCleaner: Gets rid of old junk temp files, old browser cache files and other 'crap'. You can also scan your registry file and get rid of obsolete registry keys and other problems, deactivate things in your startup folder that you don't need starting up everytime you boot up your computer etc...
There is an 'upgraded' version, but the free does quite a bit. Don't be surprised if it clears out a gig or two of junk.

Malwarebytes Anti Malware: Download the free version, will have to update it's database and scan your computer.

Spybot Search and Destroy: Finds spyware and gets rid of it.

Also, if you haven't, go to 'Start' > 'All Programs' > 'Accessories' > 'System Tools' > 'Disk Cleanup' and also now and then run 'Defragment'.

Just some things that might help. All free.