If by "eval" you mean it's one of the RC's released in 2009, you only have a month or so left before it starts shutting down on you. Do not delay -- once you see the warnings, get your data and apps off the computer right away before it locks up.
If you're a Windows user, I recommend you get Win7. It's worth it. OTOH, if you're a Mac, Linux, or Unix user, and already happy with one of those, you might want to think twice about Windows (of any kind).
Maybe I'll see if I can scrounge a copy of XP somewhere. I think iTunes still runs on it.