Microsoft Security Essentials is good enough for virus detection. Do not worry about that. I would suggest that you restart your computer and scan it with Essentials in Safe Mode. Thats where all antivirus software programs are at their lethal best. Next, you can perform some registry cleaning as i guess you haven't done that uptil now. There are numerous registry cleaners available like Reginout, CCleaner etc which work fine. But remember to always go for the paid ones as free software programs are just a scam 99% of the times. I personally use Reginout and it works well to give a much needed boost to my computer's speed.
If none of this helps then you should back up your data to a CD, format ALL of your partitions on your hard drive and then re-install Windows. And yes, consider using Windows 7. Vista itself is sort of a lagging thing and that might also be a contributing factor to why your computer has slowed down.
Good Luck.