agree with what being said here.
1 of the major factors is that winME belongs to the win9X kernel.
kernel is basically the engine behind the OS system.
Now winXP is based on the winNT kernel (which are quite different). This change alone would cause some conflicts.
You may not notice any problems at the start, but eventually things will happen and you won't understand why.
1 of the other major factors here is that you are crossing not only from win9x to winNT kernels, but also going from FAT to NTFS.
Although you can change file systems while still keeping your programs, there is no guarantee they will take to it.
It may seem like a headache to wipe out your HDD (format) and reinstall everything fresh... but in the long & short run, you will have far less headaches.
Do yourself the favor and clean install. You will also get the satisfaction of knowing how to do that and knowing your system is clean from apps you don't really use anymore, etc... .
Good luck!