Hmmm.....     I had trouble figuring that out as well, but there are several ways...  In order of preference:

1.  Check the manual for your computer or mobo and it should tell you how to upgrade the memory and what kind to get.

2.  go to http://www.4allmemory.com and they have a tool where you punch in the system you have and then it tells you what memory you need.  Then you can buy your upgrade right there as well...  

3.  Access the memory you have now and there is a tag on it with the specs.

If none of the above are an option...

4.  Hope someone else responds with an easy way through your system tools.

I use XP and all I can get it to tell me is how much memory I have, which doesn't tell you all you need to know.  I used the 4allmemory site to figure mine out.