Wrong section i know, choobs look here more often though.
Windows Vista:
Click Start, Search for "system" in the search box, Click system when it comes up.
Look Under SYSTEM TYPE
It should determine if you are 32 or 64.
Windows XP:
Click Start, Run and type in "winmsd.exe", Click OK
wait for the System Information box to pop up.
Type in "Processor" in the bottom search box.
x86= 32 BIT
x64 = 64 BIT
Windows 7:
Click Start, Right click "Computer" and then click Properties.
Click System and you will see the BIT