That's why people can rent them out for good money. Just watch out that DVD port's been cleaned well before you start turning it on, and, as always, be careful you don't get a virus as the previous user may've given it one.
You can do a Control-Atl-Delete and check the Task Manager and see the programs running in memory and see where the resources are going. If something other than "System Idle Processes" is taking 50% or more of your CPU power and lots of memory and you dont recognize it, it could be a virus or it could be some program stuck in memory. Both IE and Outlook have a habit of doing that when they crash especially in XP and Vista. A reboot would fix that though if that was the problem.