It might be that the second module is faulty. Take the good one out and place the second one in the firsts slot. If it works then the module is good. If not, it is either faulty or latency diffrences do not allow it work. Try to clean the next slot carefully and insert the good module. Make sure it is locked firmly in place. This will check the slot itself. If the computer doesn't work then the slot is no good. If you have another free slot try it there.
You shouldn't worry about different speeds because the computer will adjust itself to the lower of the two. As far as latency is concerned that is important. If you manage to run the second module but still have problems then latency should be checked against settings in the bios.
May I take the liberty and advise about your system: It is too old and slow for XP, even if you double the ram. It would run beautifully with windows 98, even with only 128 MB ram.