Vista take a large disk space after the first installation:
build 2.12 GB (2,281,192,634 bytes)
InstalledRepository 177 MB (186,577,063 bytes)
packages 510 MB (535,020,852 bytes)
Program Files 628 MB (659,145,925 bytes)
Users
Windows 4.81 GB (5,172,100,125 bytes)

Here are some tweaks can reduce the disk usage:

C:\Windows\Web\Wallpaper
Here are the wallpaper for windows, you can delete those you don’t need, or move to another disk(total 45.5MB).

C:\Windows\System32\DriverStore\FileRepository
This is the backup of dirver for windows itself, you can delete(total 1.03GB).

C:\Boot
This is different luanage boot interface for windows. You can delete those you don’t need(total 13.3MB).

C:\perflogs\System\Diagnostics
Here store the test log files for windows system testing. You can delete those you don’t need(total 9.39MB)

C:\Windows\Downloaded Installations
Some software(such as dreamweaver) decompress the installation files here when it install, you can delete it.

C:\Windows\Help
If you are familiar with vista, you can delete. But I recommend you keep it(total 105MB).

C:\Windows\winsxs\Backup
Here are some backup files, you can delete(total 353MB).

C:\Users\Public
Here are some pictures, videos for public users, you can delete(total 217MB).

C:\Windows\Web\Wallpaper（45MB）
C:\Windows\System32\DriverStore\FileRepository（1030MB）
C:\Windows\Help（105MB）
C:\Users\Public（217MB）
Total 1397MB. If you delete these and move user files and pagefile to other disk, your C disk usage no more than 5GB