I'm upgrading my sister's laptop's RAM. Being 4 years old it has two SODIMM RAM slots, only one is in use with a single 256 PC2100 DDR stick in there.

The laptop is a Compaq Presario 2100 (the SKU is the 2104EU), according to Kingston and Crucial's memory advisor services, the max the laptop can accept is 2*512MB 2700 (1GB). (http://www.crucial.com/uk/store/listparts.aspx?model=Presario%202100%20Series).

So I bought two sticks of unbranded RAM (which use the Hynix chips) of the same specification as the ones Kingston recommends (thus saving money).

But after inserting both, one, or either of the SODIMMs the laptop refuses to boot. Replacing them with the original solitary 256MB SODIMM makes the laptop work again. So either both SODIMMs are fried or they're incompatible with the laptop, even though the laptop's manual and Kingston/Crucial both say they should be.

I don't have another laptop I can check the RAM against. Any suggestions on what to do?

Thanks 

I tend to use the SMART memory advisor since they use ROHS standards on their RAM.  They show their own module plus OEM part numbers that will pull up other companies qualified RAM as well.

 http://www.smartupgradeconfigurator.com/config/index.asp?WebName=COMPAQ&Submit=GO 

Which brings us

http://www.newegg.com/Product/Product.aspx?Item=N82E16820141323&Tpk=DC390A 

Hope that helps

Thanks. But I went to a local IT shop and tried a few other pieces of RAM out. It transpires that the laptop can't take more than 512 in the form of two 256MB sticks. So it appears the database (and the info given by Compaq in the manual) is wrong.... or something happened to the laptop preventing it from holding more than 512. 

It most likely has something to do with the amount allowed per bank.  So single sided versus dual sided memory modules.  256MB per bank with two slots (each slot has two banks) would give you a 1GB maximum.  So you would need 256MB per side.  If the 512MB modules have all 512MB on one side of the module it's not able to recognize the RAM's correct configuration.