As I mentioned earlier, Vista allocates itself around 15% of your available HDD space when its installed (effectively 15% of the entire drive as no apps or data will be present at that point) for system restore files and "shadow copies" of files you have changed which allow you to roll back to a previous version. You can reduce this by following these steps.
First, to see how much storage space is currently allocated:
1. Click the start menu
2. Click on All Programs, and then Accessories
3. From the accessories menu, right click on Command Prompt and select "run as administrator"
4. Type "vssadmin list shadowstorage" into the command prompt
5. The results should tell you the amount of storage used and the maximum amount alloted
If you're not happy with those numbers (and few people are which is why Win7 uses 5%), here's how you resize them. Keep in mind, you will lose some older system restore points when you do this.. If you are at allunhappy with the way yur PC is finctioning and you think restoring it to an earleir time would help, then do tht before going any further.
1. Repeat steps 1-3 to get bring up the Command Prompt
2. Type "vssadmin Resize ShadowStorage /For=[your hard disk]: /On=[your hard disk]: /MaxSize=[how much space you want to allocate]"
3. For example, for your C drive: "vssadmin Resize ShadowStorage /For=C: /On=C: /MaxSize=2GB"
4. You should get a message saying "Successfully resized the shadow copy storage association, and you should have more free space on your hard drive.
Hope this helps.