My disc space was recently running low and a Windows "pop-up" suggested that I run Disk Cleanup. I ran it as I usually do, but then I noticed that it was compressing old files - I never have this box checked so it must have checked it by itself! My question is this - I cancelled Disk Cleanup before it went too far, but how do I tell if it compressed any files and how can I tell which ones it compressed? Is there a way to search for them? Or is this nothing to worry about...

Thanks

Compressed files and folders have a blue color (Example: C:\WINDOWS\system32\dllcache). You can turn compression off in the advanced properties.

I absolutely recommend compressing entire drives. I saved 16GB on C: when no slowdown at all. Just remember to defrag thoroughly after.

I recommend not to compress except if you're really low on disk space, then it's worth a shot.

Compression causes slowdowns in Windows XP (I have experienced that), it is time consuming and plus it increases the frequency of fragmentation.

And this is only logical.

There's a difference between theory and practice. I have a reasonable PC (3.6Ghz / 1024) but I also use it as a multi track recorder and DAW - so any slowdown or increased fragmentation would certainly have been noticed by now.
Especially recommended for laptop with minuscule drives.

Oft-changed files should not be compressed. Rarely-changed files can be compressed with impunity.

Rarely- (or never-) changed files but frequently accessed actually benefits from compression. Why? Because there is less data to be read from the hard drive, and in-memory decompression is waaaaaaay faster than hard-disk access.

Be careful not to compress files required at WinXP startup.

Compressing archived files (or compressed audio files) causes slowdown in writing; the file will not be compressed further, but Windows will spend some time analyzing the file trying to compress it. It also does not give a speed advantage when reading, in fact, makes it sliiiiightly slower.