With regard to the first question, how often do you download, how often do
you receive and open attachments on your computer...a safe user would scan
any file or attacment brough into the system regardless of the source.  You
should probably do an antivirus scan a few times a week, more if you
download files and open attachments in your e-mail.

Defrag is also a bit dependent on usage as well as how the system seems to
be working.  You don't really need to defrag all that often in XP but if you
are doing a lot of adding and removing programs, this will tend to fragment
files a little more so what watch your usage pattern but also note how the
system is funcioning.  If you aren't seeing an appreciable deterioration in
performance you are probably ok not to do it.  If the system seems a bit
sluggish, you might want to give it a try.

NOTE: one of the signs of a virus is the system running uncharacteristically
slow so if you notice that type of situation, to play safe, run a virus scan
first.  Also be sure to keep your anitvirus software up to date.

--
Michael Solomon MS-MVP
Windows Shell/User
Backup is a PC User's Best Friend
DTS-L.Org: http://www.dts-l.org/