Using XP and want to do a disc cleanup periodically.

In December 2007, MPTech asked a question about Compress old files and I could not figure out what the consensus was.

When doing a disc cleanup, lots of space is taken up by Compress old files. Do I put my check mark there so that disc cleanup will get rid of it? Do I leave it unchecked because I have lots of unused space? (I am only using 18% of my total hard drive.)

If I put a check mark there, am I deleting old files or am I compressing old files? Again, I do not need to compress, because I have lots of space.