But your parents have the minimum amount of memory for Vista. Put another GIG in for them and then test. 1 GIG for XP is just about right. Unless you have checked they probably have a bunch of junk running in the background as well.
...as far as overall speed? I just bought my parents a new computer, HP a1720n with Vista. I have a HP a1610n with XP. The Vista seems to lag a little bit, just not as snappy as XP is. Both computers have the same amount of memory, 1.0 GB. However, theirs has the Duo Core processor. Is that the reason for the lag time?