It's the eval copy that will expire this summer. I'm debating whether or not I want to upgrade to a full retail copy. All I really use it for, is to sync my iPhone to iTunes.
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).