The mmsystem.dll file controls the XP subsystem that lets you read old 16Bit DOS programs. This error is caused by trying to install or run an old program that overwrites one or both copies of this file.

Usually, one copy is still intact (68kb) and the other one is larger. You can use the correct copy to replace the bad one. (This will save having to find another XP system.) Just follow the instructions below.

Click on Start and choose Run. Type explorer and click OK. This opens Windows Explorer which basically shows a map of your drive. In the left window are the folders and the right window are your files.

Go down the left side and click on the arrow by the Windows folder. (This will expand it to show the subfolders under it.)

Find and Click on the 'System' folder then in the right window locate the 'mmsystem.dll' file. If it's the right size (68kb), right-click on it and choose copy. If not, right-click and choose delete.

Go back to the left side and click on the 'system32' folder. On the right side again find the mmsystem.dll file. If this is the larger one, right-click and delete if not, right-click and copy.

Now go back to the left window to whichever folder you deleted the file from (system or system32), right-click and choose paste.

If you find that both copies are the wrong size, you can follow the procedures above to open Windows explorer on another XP machine and copy the file to disk.

You can also extract the correct file from your XP CD if needed. You can find instructions on extracting files in Windows Help.