Do you mean your computer or your operating system?
Most computers currently on the market have 64 bit processors, but most are sold with 32 bit operating systems. It's a fair bet that if you have a 64 bit version of XP or Vista, you wouldn't need to ask the question, so I think that you can safely assume that yours is a 32 bit version.
If your Operating system is Windows XP, follow these step to know what Operating System edition you have.
> Start
> Settings
> Control Panel
> System
Here you would have all information regarding your system.
For more information go through with this http://img.tomshardware.com/us/2005/02/21/pres cott_reworked/properties.gif
To run 64bit, you need to have a 64bit CPU and chipset, as well as a 64bit OS and programs.
It's only worth running 64bit if you do one of the following:
- Something requiring more than 4GB RAM
- Lots of intensive processing of video (or similar) with less than 4GB RAM (assuming your encoding program is optimised for 64bit)