The best thing about computers - the fact that no two are exactly the same - is also the biggest problem. Software is designed around a stable working environment, and the slightest little thing can break that stability and scatter it across the proverbial floor.
Take W40K:DoW, for example. Used to run perfectly fine, then I reinstalled my OS and it stopped working right. Crashed on load every time. After a few emails back and forth with THQ support, I found out the problem: one of Spybot SD's 64-bit utility applications. The kind of thing that's never present in a developer environment, and thus may do things at the point of execution that were never anticipated in house. Disabling the offending third party app fixed the crash instantly.
EVE Online, which I play, had a long-standing problem with the Creative Soundblaster Media Center application. The two absolutely would not get along. It became enough of an issue that I spent ~$300 to build a low-end gaming rig purely for EVE Online. Didn't even install audio drivers. Ran *flawlessly*.
I run a rather minimalist setup. I don't know enough about Win7 to be able to trim all the fat from the components/services list, but I know enough. I've got almost everything disabled that isn't essential to a stable operating environment. I'm extremely careful with my web browsing, everything has to ask me for cookie permissions, I don't run third-party communications software on this rig at all (ie AIM, YIM, Skype, etc.). Steam has never given me the slightest issue. I was one of those people merrily playing Skryim in 5-hour+ stretches while reading all the forum posts of people with serious problems and wondering what they had installed to make their builds, many similar to mine, inherently unstable. Never found out, of course = P
Anyhow, the point to this little rant is that everything matters, every last factor of your hardware and software loadout changes your operating environment, and not always for the better. If you want to do a stability impairment test, disable every background app either through component services or msconfig, then slowly turn things back on and see if/when stuff breaks.