PDH.DLL stands for Performance Data Helper Library and it provides functions that are necessary for basic operating system tasks to run correctly. This file has been the subject of various virus hoaxes but it is a legitimate Windows file and should not be tampered with.
Should you find your computer malfunctioning, or even refusing to boot up, you need to take steps to fix this file immediately. The recommended option is to reinstall the latest Windows Service Pack Alternatively you could try to save time and copy the file directly from the CD but this is NOT recommended. After this procedure it might be wise to re-install Microsoft Office in case of file corruption and to run a registry check utility program to fix any remaining errors.