To jednodušší z nabízených řešení fungovalo i u mě:

Anyone getting the "Disk Defragmenter Could not start" message and on Windows XP Home This is what worked for me. Go to Control Panel, Select System and select the Advance Tab. Now Select Settings Under Performance and again select the advance tab. Under Virtual Memory select Change. Make sure that the Paging File Size for selected Drive for the Drive partition you want to Defrag has either a custom at least 2 times the amount of installed RAM if the space is available or is set to System Managed Size. Once I did this Defrag started working again. I don't know how the Page file got turned off on my system but this has been driving me mad for months!!

Česky: Nastavení -> Ovládací panely -> Systém -> Upřesnit -> Výkon -> Nastavení -> Upřesnit -> Virtuální paměť -> Změnit Velikost určí systém -> Nastavit -> OK

Po restartu začala defragmentace fungovat i u mě. DisKeeper zřejmě virtuální paměť vypnul.