Windows virtual memory is called a Page File. In Task Manager, under Performance, you can actually see the amount of the total page file in use. Windows stores the pagefile in a file called, umm… pagefile.sys that is a hidden file located on one of your hard drives (by default C:). If you enable hidden files to be viewable in folder options, you will see it sitting there.
As mentioned above, you can manually set it to different values if you know what you are doing. I wouldn’t recommend messing with it if you are unfamiliar with it. Truth in that for best performance, it should be located on your fastest (though not necessarily your largest) and/or most idle hard drive and even better if it resides in it’s own partition to prevent disc fragmentation.
The page file by default can change size as needed, but to prevent fragmentation, it is often wise to set a fixed page file size. The general rule of thumb for page file maximum and minimum sizes is 1.5x the amount of installed RAM on your system. I would NOT recommend creating a 50GB swap file.
I have my page file sitting on my second hard drive (non system disc) under it’s own partition set to 3GB in size (2GB RAM).
Note, this applies to Windows XP. I believe that there are a few changes in Vista with regards to how virtual memory is handled so all of the above may not apply.