From Windows XP Inside Out:
tip - Shut down "low disk space" messages
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When you’re running low on disk space, Windows displays a warning message in the notification area at the right of the taskbar. Clicking on the balloon opens the Disk Cleanup utility and allows you to clean out unwanted files. If you’d prefer not to see these messages, you can disable them globally by adding a value in the registry (unfortunately, this option cannot be used only on specific drives). Open Registry Editor and select the following key:
HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer\
Choose Edit, New, DWORD Value, and name the new value NoLowDiskSpaceChecks. Set the data value to 1. Close Registry Editor to apply the change.
Windows will no longer display these warnings.