Windows only sees 768 MB RAM out of 4 GB
Open the Device Manager and under the View menu, choose "Resources by connection". Expand the Memory node and see if you can figure out what, if anything, is occupying the memory. If you're only seeing 768MB, there's probably an entry starting at 3000 0000 named "PCI bus". Expand that node, and see if there's anything actually at 3000 0000. (RAM is "System board".) You can then try disabling that hardware, perhaps installing an alternate (video, network, drive controller, etc.)
Start → Run → type msconfig and click OK.
Open the BOOT.INI tab, click Advanced Options..., and see if /MAXMEM= is checked.
Other than that, shut down the computer and reseat the memory modules. Or remove the modules and try various combinations. Also run a check with Memtest86+.
768 MB could be just 1 GB working OK, minus 256 MB shared video memory.
Some ideas of what to try:
- Clear the CMOS
- Unplug the power and remove the RAM for 30 minutes, reseat it well.
- If you have access to another identical computer, try exchanging RAM.
- Take the computer to a repairman that will have the tools and spare parts required to run all the needed experiments.