I'd advise you don't even touch the virtual memory settings like Paulst30 is suggesting. It's an easy way to get into trouble.
Check these details and tell us what the numbers are:
RAM -- found by right-clicking my computer and clicking properties from the menu -- it'll be at the bottom. 512MB or something like that.
hard drive -- go into my computer, and you should see your hard drive, shown as C: or similar (I assume you have only one. if more than one, C:, D: etc., then do it for each of them). right-click the drive icon and choose properties, and write down the free space it quotes there.