Well, I have 2 parts to this question:
Externally:
Every 4 months or so, I shut my computer down, pull the screens off and blow out all the dust. I wipe it down, make sure all the cord connections are tight. And I blow out the fans. I blow out my keyboard, and wipe it down as well as my mouse. I clean off my mousepad. And I wipe off my screens and check their cord connections. I also take a moment to make sure all my cords are plugged in to their outlets well.
Internally:
Yearly: I go through my "Follow Up" folder and delete anything no longer needed. I check out the "File Cabinet" folder and see if anything can be deleted. I move all my scrapbook pages to my external hard drive for filing.
6 months or so: Run the Disk clean up. Usually I run a malaware checker just in case.
Monthly: I move all my digi kits over to the external hard drive. Some may get deleted, just because they were super specific or I used the kit a bunch and I'm good letting it go. I do try to move things more often, but monthly is my minimum. Check my scanned documents for things to delete. Check my online backup and server backup and that it's working correctly.
Weekly: I go through my download folder and see what should be filed or deleted. I try to keep that up daily, but weekly is my minimum. I clean up my desktop. All those extra icons and such that I don't need. Empty my recycle bin. Delete zipped folders.