The W2k install process will ask you, at some point,
to insert the CD for an eligible upgrade system. Once
it sees that, it'll tell you to reinsert the W2k CD,
and continue happily.
Also dont forget that if you need to save your data you
can. At some point during the install you will get a
format option, if you need to keep your data choose not to
format and save existing data. If you are trying to fix a
problem with a clean install somtimes its best to format,
nuke it from orbit just to be safe.
PCC