I have a dimension 5100 desktop computer i want to

All you need to do is make sure you have the correct type of RAM (SDRAM, DDRAM, DDR2, DDR3).

You can use Newegg's memory configurator to find out which type of RAM you need using this link :

http://www.newegg.com/Product/ProductConfigurator.aspx?plf=10&category=17&name=Memory-Configurator

Also, remember that some motherboards can not handle more than 1GB of RAM.

No, just make sure its compatible with the motherboard. Power down (unplug the power cord - just being safe).
Remove side case, install RAM and power back up. It should tell you that the configuration has changed, confirm (it doesn't always ask).and it will recognize the RAM.

Hi, if i assume you mean 2gb rather than 2mb. This will not be possible as i believe the 5100 will only take sticks up to 1gb. However that model has 4 slots so you could go with 4 1gb sticks, you wont have to do anything on the computer just pop out the old ones and place in the new.
One thing to be careful of is the type and speed of ram that is compatible, apparently the 5100 can use DDR2 either 400-MHz or 533-MHz.

Hope that helps

You may want to check this link for instructions and tips on installing memory on Dimension 5100.