66Mhz DIMM's don't fit into 100/133MHZ DIMM slots as they run on 5 volt instead of 3,3v.

133MHz boards can use 100MHz DIMM's but switch ALL the banks down to 100.
100MHz boards can use 133MHz DIMM's if their SPD chip is coded to use both frequencues

try the following link
www.memtest86.com

to find out what is wrong with the memory, and search fo some memory manager which tells windows which adresses to exclude.

If it's a LINUX based machine, the website gives some hints for a certain program which tells the kernel to skip certain memory blocks...