Old user accounts - use the user profiles from the system properties to delete them so you wipe any cruft from the registry as well.
Log files shouldn't be an issue to delete as long as they're not open.
Compression can help. Slows a little, especially if short on RAM, but can help.
Turn on "show hidden files" and in the Windows (or windows system) folder there should be a huge bunch of hidden compressed files used to roll back changes to updates. I usually don't have a problem "deleting" them as long as I have a backup in place and don't think I'll be rolling back any system updates in the future.
Suggestion - get your hands on an external hard disk and instead of deleting things, move them to the other storage. If there's an issue where the server goes kaplooey in the next day or two, you can restore it rather easily. Just don't use FAT on the external storage as large files can have issues with it.
I usually use 7zip to compress things I'm not sure that I'll need for awhile and archive it. Free, fast, can create self-extracting executables and tends to be better than ZIP.
Double check your windows components have just what you need installed. No need to have games if you don't use them (on a server??)
Double check that your program files subdir has just the programs you currently run in there. I have deleted old subdir cruft that was left behind from uninstalls.
Check that there aren't multiple versions of Java installed if you don't need them.
Check for caching of crud. Temp internet files, temp folders used for downloads, and cache directories in user subdirs are candidates for checking. I use "bytecount" (freeware) to get a good estimate on sizes of subdirectories because it's uber-tiny and simple to use.