You probably have a combination of Spyware/adware and too many programs that start up.
So what you can do is: (just google most of them)
Download a AV if you don't have one(antivir or avg)
Download AdAware SE, Windows Defender, Spybot S&D
Ewido
Bootvis
Ccleaner
Advanced Windows care.
(UPDATE ALL SCANNERS)
(and a free trial defragmenter like Diskkeeper if you want)
First off, you should do a clean security sweep, so after you downloaded all of this disable your system restore by rightclicking on my computer and then going to the System Restore and disabling it. Next reboot in safe mode and run Ewido, AVG/AntiVir, Windows Defender, Adaware, spybot.
This should generally wreak Genocide on any spyware, removing all those that might be slowing down computer. Just remember to run ALL of them, because they each have a different definition list so what one misses the other gets.
Next we're go into Spybot or Ewido. for Spybot to use the required features your going to have to enable advance mode and click on tools. Your going to select startup list on either one of them and disable start up programs that you don't need. (like quicktime, real ect.) now restart and see how that feels, make sure to make a Bootvis log before you restart.
Now that you've started up and defragged your startup with Bootvis start up Ccleaner and run the cleaner, then run its registry cleaner(make sure to make a backup)
Next run windows Advanced care. This is a free program that tweaks Windows and fixes inconsistancies that can cause slowdowns.
AFTER you've done all of this your going to run that Defragmenter, or use the mediocre one that windows is installed with(just startmenu->accessories->system tools->Defragmenter)
This will probably take several hours...
You can also search for tweaking guides for making registry changes that'll speed up windows, but this is a pretty easy newbie guide that won't inadvertantly harm your system.