If your system is detecting maximum RAM installed during the hard boot memory check, then that shows that the RAM chips are OK - using them is a slightly different matter.
When you viewed the RAM figures in controlpanel/system, was the 15MB the INSTALLED figure or FREE figure?
Although the vast majority of modern PCs auto-detect RAM, some do not, and require and entry and exit into/from BIOS. This is usually achieved (look at your handbook) by pressing del or F2 during initial boot. DO NOT alter anything in BIOS with which you are unfamiliar!
It may help if you confirm the basics of the PC you are using - Make/purchased from, clone, 486, Pentium 2/3, and RAM type e.g. PC100/133 etc. May be of use - hard disk size and amount taken up with installed apps.
Will assist further if I can.
Cheers
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When I was on motor patrol...