Steam was out long before windows vista & 7 came out. I used to run steam on XP back in those days.
Ah, looking at their
site, requirements are:
System Requirements
Windows XP, Vista, or 7
512 MB RAM
1 Ghz or faster processor
Intel Mac, OS X version Snow Leopard 10.6.3, or later.
Two-button mouse strongly recommended
1GB HD space (recommended)
Internet connection (broadband recommended)
Your PC will still need to meet any game requirements for the games you get through steam.