Lots of stuff at play here. Essentially: it's time to upgrade your computer.
But! There are some things you can do to breathe life back into your machine for a few months to a year.
Get Wize care:
http://www.wisecleaner.com/wisecare365.html
Have it clean your computer, defrag your HDD, defrag the registry, optimize memory. Then have it check out what's on your start up programs, follow their recommendations.
De-install programs that are incentivized to make your PC run slower (iTunes, QuickTime, Bonjour, anything Apple, sadly)
And finally:
Spy-Bot Search and Destroy will get rid of most if those pesky Trojans.
http://www.safer-networking.org
Good luck, it's never fun and you should plan on spending a few hours doing this.
Remember to create a system restore point before starting.
Best! And please report back.
Edit: other advice on this thread is also very sound.