Yes. Each install is like a different computer and must have its own protection, it is starting from scratch again.
I found that SuperAntiSpyware would not run on Windows7 (don't know if they have fixed that yet), but I put MalwareBytes on with Win7 and it works fine. I use Avast, WinPatrol and ThreatFire successfully on Win7.
You could check out your Device Manager to see if any drivers need updating as well.
I really like Win7 and will be installing Ultimate on all my computers over the next few days. Have been running the RC since June and found that I hardly ever booted back to XP since then.
Edited by rowal5555, 28 October 2009 - 04:37 PM.