I would also add backup, virus/spyware scan, software/drivers updates ...
It all depends on your habits ... From once a week (many installs/downloads) to once a month.

trapper_s_john,

I run Disk Cleanup daily and Defrag weekly, both as scheduled tasks during period when the computer is unattended. Chkdsk is run monthly, at most.

Thanks for the reminder, it is time for my semi annual defrag.  What I am trying to say, in a lighter vain, is I never have run disk clean up.  I run chkdisk only when I suspect a problem, which  is about once a year, and I defrag infrequently.  I typically have over forty percent free space on my disk and defrag never has had any effect on my systems performance.  P.S. I have 1 gbyte of ram so I don't do much swapping/paging. 

Same here, i defrag HD weekly, and virus scan + spyware scan weekly, and Chk disk 3-4 weeks if i remember. Again it depends if you have been doing large amounts of installing and moving/deleting large files then defraging may be done more often.

Thanks for the replies everyone. Appreciate it!

 I was used to doing basic maintenance at least weekly on my old HP with 98SE to keep it running. Now, I gather that bi-weekly on XP is good enough. Maybe down the road I will even stretch it to monthly.