How can I find out how much RAM my Windows XP computer has?

I want to add some RAM, and want to know how much I've got and how many unused slots I have available. Going to control panel and device manager doesn't help. Maybe I'm looking right at it in device manager, but it doesn't explicitly name the chips that provide RAM. I have an older Gateway w/... show more I want to add some RAM, and want to know how much I've got and how many unused slots I have available. Going to control panel and device manager doesn't help. Maybe I'm looking right at it in device manager, but it doesn't explicitly name the chips that provide RAM.

I have an older Gateway w/ 1.8 GHz Pentium 4.

I welcome any advice.
Update: For some reason, I have no My Computer Icon on the desktop.
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