When Windows detects that one of your hard drives has too little free space, a warning appears offering to run the "Disk Cleanup" utility. Here's how to turn off this message:

Windows XP

Run the Registry Editor (REGEDIT.EXE).
Open HKEY_CURRENT_USER\ Software\ Microsoft\ Windows\ CurrentVersion\ Policies\ Explorer.
If it's not there, create a DWORD value and call it NoLowDiskSpaceChecks.
Double-click on NoLowDiskSpaceChecks, and enter the value 1, and press OK.

Windows 98

Open the Disk Cleanup utility (run cleanmgr.exe, or go to Start -> Programs -> Accessories -> System Tools -> Disk Cleanup).

Select the drive on which Windows is installed and click Ok.
Choose the Settings tab, and turn off the If this drive runs low on disk space, automatically run Disk Cleanup option

Click Ok when you're done - answer any subsequent prompts as desired.
You may have to repeat this for all your hard disks (if you have more than one)