Post Mortem worked fine on XP on my computer with no tweaking at all.
The Frankenstein game and Mummy Tomb of the Pharoah (I think someone asked about them earlier) loaded fine and played fine but I haven't finished them--not because of a system crash or anything, but because I got distracted by other games.
With Tex Murphy Overseer Director's Cut I was able to play to almost the very end in XP and then near the very end with the room with the maze-like floor (I won't spoil any further) it crashed and I have never been able to fix that. I figure I will load it someday into my computer with WIN95 and play it there.
I don't think anyone mentioned this, but I played Black Dahlia fine on XP.
I was not able to play Ripper on XP and had to load it eventually on my computer with WIN95 and it played fine there. (just in case someone is making a list). As for Amber, I had absolutely no luck with XP with it and I am thankful I saved my old computer, just so I could play it (It is my favorite all time game!)
Starship Titanic works fine on XP for me. It is really hard to tell which older games will work--you just have to pop them in and try them. For example, I thought I would give Kings Quest VII-The Princeless Bride a shot and it worked like a dream on XP.
Good luck and happy gaming!
Beth (p.s. Make sure you load all of the updates for WinXP when you are alerted because I have found that they really help. Of course, I got this computer a year ago so a newer computer might have all of the older bugs fixed when you get it.)