5 Free Memory Test Programs
http://pcsupport.about.com/od/toolsofthetrade/tp/memorytest.htm
Memtest86
Windows Memory Diagnostic
M2K MemScope
Memtest86+
DocMemory Memory Diagnostic
RAMCHECK LX Memory Tester
http://www.innoventions.com/products.htm
hardware-diagnostics
http://www.uxd.com/hardware-diagnostics.shtml