Avast free edition together with common sense is guaranteed to keep your box free from viruses. Microsoft Security Essentials is also effective and it's nicely integrated with Win7/Vista.
In your case, since your computer is booting slow, you most likely have spyware/malware. What I suggest you to do is to first delete all temporary files from your computer, run SuperAntiSpyware and Spybot in safe mode. Then run smitfraudfix in safe mode.
Finish it off by checking your startup processes and un-tick anything you don't need (Start -> run -> 'msconfig' -> startup).
Feel free to PM me and I'll write you a quick step-by-step guide.