Computer missing RAM

i has a computer with 4GB of RAM installed.  the POST tells me that it detects 4GB of RAM.  Windows XP tells me there's only about 2.4GB of RAM though.  which sucks, since that's 1.6GB of RAM less than i should have.

WHY?!?!

Install XP 64-bit. It should be able to detect and use the missing ram. Since your XP is a 32-bit edition, it can't use more than 2gb of ram or so.