I defrag less often than I used to, now, but download/move/delete files fairly often, trying out different software, playing around with tweaks etc. So about every two weeks I do a boot defrag with Bootvis, and a full defrag with JKDefrag. This is following a scan/check disk. CCleaner gets used at the end of every browsing session. Avast! and SpywareTerminator auto-update, Comodo Firewall, Firefox and Windows are set to advise me of updates but don't download them, all other applications (SAS, Asquared, AVG AS,SPybot, PG2, the printer, the camera editing program, Java, and Flash player, I update about weekly or two-weekly. Error messages generally only occur following an ill thought out tweak, and are usually explainable. Those that aren't, or recur, I ask for help on a forum like this one. Why should anyone put up with frequent error messages/things not working as they should?
Every two weeks or so it gets a full virus scan, one or two full spyware scans, and a rootkit scan.
I used to be a bit obsessive compulsive, I guess. Now I'm much more relaxed about it. I believe that to learn to un-f### something, you have to partly f### it up a bit first.

[Edit] my computer runs fine, too.