Go to Start (Is it still actually called that officially in Vista, since it's now just a logo), right click on Computer, and select Properties. For me, I see at the top a section called Windows edition, and mine says
Windows Vista Business
Service Pack 1

While SP1 is not a miracle cure all for Vista (just got done reseating network cables and trading ports to see why mine was dropping 80% of the packets to the network), it is slightly better and worth getting. If you don't have it, go to Start, and type in Windows Update. I don't think SP1 is going out over automatic updates yet, but it will show as a choice if you run the updater manually.