When you're dealing with a computer you do not own or has had many hours of use prior to your ownership, it's anybody's guess as to what went on before with it. You just don't know why myriad of software conflicts or infestations may have plagued it, either now or in the past. Sometimes a conflict won't reveal itself directly. Services will compete for the same resources and bring the system to a crawl, without any warning from the operating system. However, I've heard that W7 is supposed to be a little better at identifying such conflicts. You can still troubleshoot within Vista, but it's a more concerted effort.
Anyway, sometimes starting from scratch is best. Glad to hear that the reformat and new installation solved it. But like you, I'd be burning to know just what it was that tripped up the computer before. You know, this would be an excellent case for a kind of operating system configuration snapshot and compare utility. It gathers up all information about services loaded, installed drivers, user customization, and installed hotfixes. This is saved and is then capable of being copied to any other installation (on another computer or a new installation on the existing computer) where it can be referenced for comparison.
Also, I was once advised by an IT guru friend from the past who once told me that no matter what, it's always a good idea to completely re-install a Windows operating system every 4 years. Something about it degrades over time (registry settings, system file management/configuration, hard disk internal file tables, etc) and it's far more trouble free to just wipe the slate clean and install fresh. That was back in the days of Windows 95. Even now, it seems like it's still a good idea to do it with Vista or W7.