Can I install CoD on more than 1 PC?

[SEE]

I'm new to Steam but having installed the game initially on my XP partition then adding a Win7 64 partition and installing to that I found that the original XP install would no longer work even in 'off line' mode. I had various pop up Steam warnings and options but got the distinct impression that whatever info steam holds my Win7 install would have been automatically de-activated - I wasn't prepared to take the risk so deleted it from XP in the end.