Windows XP media edition, 2 gigs RAM, no large address aware thingamy.
Actually the simplest way - but most time consuming - is to start over from scratch, unzip the 1.4 backup, delete the program files\ubisoft folder, replace with the original unmodified (contains no artificial sweeteners or preservatives) SH4 1.4, then hunt up the amazon thing and reinstall 1.5.
As for multiSH4, it may or may not be responsible for more frequent crashes, but since I never installed it in the original patched to 1.5 version - in fact not even on the same disk, the only place I used multiSH4 was on the F drive - it's unlikely to have affected the original in any way. In fact I never actually played the original until recent testing, back in 2007 I copied the folder to a different location before I even tried the game.