why does my computer say Warning - one or more memory DIMMS are out of rev?

Guinia 1,
I am sure they meant well, but the error message means that the newly installed RAM is not compatable with:

a. Your computer
b. The other DIMM already installed
c. The system detected that one or more of the DIMMs are not the correct revision and the DIMMs do not meet the Intel® SPD 1.2 specification or 66-MHz DIMMs are installed.
or even
d. That you or someone else mishandled the DIMM and blew the chips due to ESD. Electro Static Discharge, is one of the few things an individual can do to damage or destroy his or her computer or hardware components. Much like the shock you receive when rubbing your feet on the carpet and touching something metal, ESD can occur when working in your computer and will cause components you touch to no longer work properly. ESD can occur without the user feeling a shock and will only occur while working on the inside of the computer or handling an expansion card.

In any event, you need to remove the newly installed DIMM.

Here is a Memory tutorial that you may find useful:
Kingston - Ultimate Memory Guide
http://www.kingston.com/tools/umg/umg06.asp

Crucial has an online memory scanner that determines your computer configuration and gives you the exact information as to what memory module specific to your system requires. Just click on the blue button located to the right to the bottom of the first page "Scan My System" IMHO, I now buy Crucial as it they have the most reliable RAM, for really reasonable prices. They give a 30-day guarantee after installing the RAM so that, if you're not happy, you'll get a full refund.
http://www.crucial.com/

Duh. Take it out.

Seriously, there's probably a mismatch between the type/speed of the memory.

try re-arranging the dimms. check to see that they are the same speed (pc100 or pc133) some pc133 will not work at pc100 speeds but some will... i have never seen this error message so i am going to use google to see what pops up... here you go: 'System detected that one or more of the DIMMs are not the correct revision. DIMMs do not meet the Intel® SPD 1.2 specification or 66-MHz DIMMs are installed.' That's from the forum in the link below. Google is the BEST source for finding answers to technical questions as 99.9% of all computer problems have already been solved!

The computer will NOT blow up!

As your's is a very old PC...may be your Mother board is not accepting those type of DIMMS.....

Talk to any Hardware engineer..they will resolve your problem...

Not all Motherboards accepts different types of DIMMS (I mean speed, make etc...)

Good Luck..

Your memory are not sync. You need to buy a new pc, or since it is just a warning, you can ignore it until it blows up.