System folders which you can empty to save you diskspace:
C

WINDOWS\Temp
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WINDOWS\Prefetch
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WINDOWS\Downloaded Installations
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WINDOWS\system32\dllcache
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WINDOWS\Installer (in this case you have to check which installers you don't need and then delete them)
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WINDOWS\Help (this solution is optionally, if you don't need system help you can follow this tip)
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Documents and Settings\(*your username*)\Local Settings\Temp
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Documents and Settings\(*your username*)\Local Settings\Temporary Internet Files