I depends on what os your running windows seven or xp. If your running windows seven:
Then Click Start, type system in the Start Search box, and then click system in the Programs list.
Its say 64 bit or 32 bit there so you can see it.
If you have xp then you can go to start, run and type in 'sysdm.cpl' and then goto the general tab and if it says Windows XP Professional x64 Edition Version then its 64bit but if it doesnt have 64bit then its 32bit.
got it from the Microsoft website and a bit from myself