If you have the Dell service tag number for the machine you can go to Dell's web site, support, and type in that number after you've navigated to the appropriate screen.  It will then tell you everything you ever wanted to know about your machine.  Another way would be to do a google search for "Dell 4100" without the quotes.  I'll be you get a lot of hits. It might be faster to look.

>Hi,
>How can I tell, with out opening up the PC case, how many of my memory  slots
>are taken up?  Also, how can I tell what type of memory I need to  buy?
>I currently have 512mb of RAM and want to install more.  Whether it  matters,
>I have a DELL Dimension 4100, running Windows XP Professional.
>
>

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