How much memory can I install in my CompaqP866 Deskpro EN Windows XP updated 1/3/2013. Currently shows 192 MG Physical Memory.
Compaq Deskpro EN P866 supports upto 512MB using "two 256MB" modules. It has only 2 slots. You can search for "PC100 / 133 SDRAM DIMM" Non-ECC memory. I found a site which has the memory for your system thought it might help your pursuit of obtaining the right memory: http://www.memorystock.com/memory/CompaqDeskproENP866Minitower815e.html
P.S: Upgrading will increase the overall performance but the bottleneck is your processor. I think it is time for you to consider buying a new PC.
Anyways, good luck with the upgrade.