According to what a gamer said in the Frogwares forumhttp://www.frogwares.com/forum/viewtopic.php?f=51&t=57537
I had the same problem (my game would freeze even not at Whitechapel or at the prison, but at random locations).
What solved the problem for me was to turn all options to minimum. Water, shadows, bloom, depth, everything off. Anisotropic filtering set to none. Then I got no more problems, and even the lag that I had went away, and the game ran very smoothly.
There is also a patch for HDMI problems ***here***
, but unless you're using an HDMI cable with your monitor, I don't think it addresses your problem.
For Scavenger Hunter, the "too big for the monitor" problem may be related to using a widescreen monitor. If you have a spare Windows disc, you can try playing it in Virtual PC on your Windows 7 computer. Maybe your son could set that up. There are instructions for using Virtual PC 2007 with a Windows 98 CD at ***this site***
, but you could also use a Windows 2000 or XP CD. In Virtual PC, you should be able to play the game windowed.