Search your disk for .hdd You should a large file. You can delete the folder that contains that file as it should have some other files related to your install like .mem .pvs, .sav, Windows Applications, etc.
If you see all those and the folder holding them is called something like "Microsoft Windows Vista" then remove the whole folder.
Please note, once you remove it, it is gone. If you activated Vista then you will lose this activation and will probably have to talk to MS to get them to activate your copy again on a different machine and they may refuse.
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