Help Switches on motherboard for memory????

[lostmymind]

Please let me thank in advance any and all repliers...I'm a newbee at these forum things. My problem is this.. I have a 550Mhz P3 on a motherboard (maker unknown at this date) When i purchased it in june 2000 i could only afford 128 pc 100 simm. Recently i purchased 3 256 pc 133 simms. I removed the 128 and installed 3 256's.. Everything was fine for a week,then it looked like i had a virus on the machine with all kinds of stuff getting screwed up..Bought Norton's Suite..No viruses detected..System diagnosis picked up memory address failure..tested each simm individually,all passed.. then used one to test all 3 slots individually,all passed..when i put a second simm in i got address failure (different address locations for different simms)I need to know if there are dip or pin swithes on a motherboard that allow memory to be accessed correctly(if so who do i contact) or if there could be a 256 limit on my board(why 3 slots then).If there are any other suggestions you might have i would appreciate them.