If I had (and I have worked on) a computer that seems all fouled up, assuming I can still get it to start, I would do the following things:
START --> PROGRAMS --> ACCESSPORIES --> SYSTEM TOOLS -->*
*first, do Disk Cleanup, and tell it yes to all (compress some, delete temp files, etc.)
*second, do Disk Defragmenter
After this, I would go to Microsoft and make sure I had downloaded and install all of the fixes and updates for my software.
Then, I would use my big three: Spybot Search& Destroy, Lavasoft's Ad-Aware, and Spyware Blaster. Load each (they're free), download their updates, and run them one at a time. Do a full scan with each. Get the crud out of the computer.
If you can't load these chances are your firewall is bonkers. Like I said, I think you should uninstall and reinstall your firewall. It's like using the "reset" button on things--once it gets rid of the garbled memory it starts from scratch and is meant to recognize what you're using. So even if you get the above programs to work and clean things out, you still might want to redo the firewall.
If all of these things don't work, on repeated attempts, then comes formatting the hard drive and reinstalling stuff. EVEN IF THE TECHIE AT DELL SAYS TO FORMAT YOUR HARD DRIVE, DON'T DO IT!!!! THIS IS OFTEN AN ANSWER YOU GET FROM A TECH WHO DOESN'T KNOW WHAT HE/SHE IS DOING AND THEY SHOULDN'T BE TELLING YOU THIS. Capiche?
There are a lot of things to try before formatting a hard drive. It's the last resort, before replacing the hard drive. (No one may tell you to do this, but telling you to remove your hard drive sounds as hair-brained as telling you to format it, is why I bring this up).