need to change cpu voltage

Changing processor voltage may involve BIOS settings, physical jumper changes and/or processor mounting pin masking or revealing, or some combination thereof ... depending on your processor, socket, motherboard, and chipset. However, without more info than you've provided, I'll say I doubt processor voltage is at the root of your problem.

No guarantee here, but it might be useful to know the exact model number of your machine and its processor details, and the type and amount of memory your machine has, along with what OS it originally came with. It may not be practical to bring that machine up to date. If its a relatively older (Win98 or earlier, PII/equivalent or earlier) IBM machine, w/128MB or less of at least PC100 memory, it can be a real challenge to upgrade the OS - as I recall, IBM's own recomendation from years back was to use the machine's factory restore disk to revert the machine to as-shipped condition, then over that do a full install of the OS to which you wished to upgrade ... and even at that there was all sortsa griping out there on the newsgroups and discussion boards indicating lotsa folks found Aptivas uncooperative when it came to upgrading from Win95 to Win98 or Win2K, and just about impossible to upgrade to WinME or WinXP. And even when the upgrade "took", finding compatible drivers for some of the stuff IBM put in those machines was a trick in itself.

I sincerely doubt upping the CPU core voltage would solve this problem. Also, if you found nothing about voltage in the BIOS there's probably not much you can do about it anyway.

One possible cause for this problem may be that the RAM in your computer is broken. I suggest you find yourself a memory test program on the web. These usually can't be run from Windows, though. They typically require that you download a floppy disk image which you have to write to a floppy disk and boot from, or that you burn a bootable CD.

If you have a good Internet connection and access to a CD-burner you can download "Ultimate Boot CD" from http://www.ultimatebootcd.com/ and burn it. It contains many useful tools, including memory test programs. Once burned, you simply boot from the CD and use the menu system to start the appropriate program.

If you don't have a good connection and/or a CD-burner, but have a floppy disk drive you could try out this: http://www.memtest86.com/