That machine has other issues...although 512 won't be blazingly fast it is more than enough to run XP fine with no problems.
This also would not account for the extremely slow bootup time.
If the virus scan took a long time it could be because it was the first run on a new install but it still sounds slow to me.
How much free space is on that drive ? ?
When's the last time chkdsk and defrag were run ? ?
What prompted the re-install to begin with ? ?