PowerDVD is indeed a good choice.
I think it (and others) can be found at www.digital-digest.com/dvd
Win98 also has a dvdplayer. It can be installed like other windowscomponents.
If you don't find it try extracting it manually:
extract -a d:\win98\mini4.cab dvdpl*.exe
with d:\win98 the path to your win98 install files
it should be a "stand alone" exe