Originally Posted by
The Stig
Can you check your Services list and see if Windows Update is listed as "Automatic (Delayed Start)" (something like that, I forget the exact name)
One of the Windows Vista PCs I've been handed for "inspection" had a similar issue. It was quick on paper, no bloat ware (Q9000 CPU) and a normal hard drive. It would start up quick, and then as you think it finishes, it suddenly runs a bit slow/laggy. Further inspection revealed Windows Update sevice (one of the many svchost.exe sub processes) was using up 600~mb memory just checking for updates.
Before anyone goes "lolvista" I've seen it happen on Win7 PCs too. No where I've found on the internet gives a specific reason why and I'm also unable to tell. Disabling Windows Update did the trick, but it's more of a workaround. However, I instruct them to seek out updates at an idle time per week (i.e. during dinner or when you need to afk for a while).