As DosFreak said, limiting or removing the amount of memory available helps in those situations, try with 64MB RAM at least. Other utilities that can limit the amount of memory like HIMEMX are XMSDSK (a ramdisk driver) and LIMITMEM.
BTW, Alien Trilogy speeds up way too fast on several parts with fast CPUs like the PIII and the GeForce FX 5500 video card won't work well with hi-color mode. At least there is the Throttle utility for slowing down that computer. It isn't perfect, but it works in this case and make sure that the -C command line switch is entered so the CPU cache won't be disabled which would make it way slower than usual. I recommend this variant of Throttle if your motherboard is ASUS branded: link.
VIA C3 Nehemiah 1.2A @ 1.46 GHz | ASUS P2-99 | 256 MB PC133 SDRAM | GeForce2 GTS 32 MB | Voodoo2 12 MB | SBLive! | AWE64 | SBPro2 | GUS