Clean harddisk when low disk space in windows vista

Simple cleanup procedures can eliminate unneeded files, freeing up disk space for other uses. Delete personal files, such as music or video files. Music and video files, such as movies or video files created by webcams, can consume large amounts of disk space. Make sure to turn off the webcam when not in use and delete unneeded video and webcam files.

Remove unused programs

• Click Start Menu
• Go to Control Panel
• Click Uninstall a Program
• In the Programs and Features window, select the program you wish to remove
• Then click Uninstall.

Remove unnecessary files
Use Disk Cleanup to free up disk space and help your computer run faster. It removes temporary files, empties the Recycle Bin, and removes a variety of system files and other items.
From Start Menu point to All Programs =>Accessories =>System Tools, and then click Disk Cleanup. In the Disk Cleanup Drive Selection dialog box, in the Drives list, click drive C:, and then click OK. In the Disk Cleanup for C: dialog box, select the appropriate files to delete, and then click OK. Click the More Options tab for more information and click Delete Files. Follow the same procedure by selecting other drives one by one.

Run Disk Defragmenter
Disk Defragmenter combines the volume's free space, making it less likely that new files are fragmented. To run Disk Defragmenter from the Start Menu:

• Click Start Menu
• Click All Programs =>Accessories =>System Tools
• Then click Disk Defragmenter
• Click Continue in the User Account Control
• Then select Defragment now

CCleaner is a much more powerful application that can delete a whole lot more from your system. And, if you use browsers such as Firefox, it can help you even delete the cookies that are associated with Firefox, which of course Disk Cleaner is incapable of doing.

I know this is an old post but if you are a user who is having the same problem you can try the following also.

Remove unwanted system restore points or change the amount of disc space used for System Restore.This Video is a tutorial on how to do that have a look.