Help needed o computer's recent performance, please!!

OK for the love of God someone has to help me, here's my problem:

These past few days, I've been having loads of trouble with my computer:
- Extreme lag in most games that used to run smoothly
- Porn pop-ups at every Windows Start-Up
- Apparent Spyware removal pop-ups (yes, I have a damn pop-up blocker!)
- Sudden freezes for a couple of seconds
- Internet Explorer takes 10x more time to open up
- Video clips take hours to buffer and cut while im watching (which didn't
use to happen)
- Programs freeze and require task manager to close

Yeah, all that and a couple of other whole lotta junk! PLEASE someone have
an idea what to do. I have already tried these things:

- Disk clean-up everyday
- Defragmented twice
- Uninstalled everything I have downloaded recently (includind Google
Toolbar with pop-up blocker because it wasn't helping me in whatsoever way)
- Freed disk space my deleting most of my games and music
- Changed home page to about:blank in hopes of no pop-ups on IE start
- Removed spyware detected by SpyBot Search & Destroy (which has never
failed me to date)

So, as a human being, I am BEGGING someone to give me an idea. If not, only
solution is throwing my computer straight outta the window, and that's
something I'd keep for the end, because I got the whole summer ahead of me,
and without a computer, my life is... meaningless...

Thanks in advance, I hope to get as many suggestions as Microsoft lets you
guys put on this thread! Peace.
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str33trul3r