The file needed to open the User Accounts applet is nusrmgr.cpl located in the Windows\System32 folder.
It may have gotten corrupted or misplaced or wandered off but you can extract a new one.
1. Go to Start>Run, type in "msconfig" (without the quotes), click OK.
2. On the General tab, click the "Expand File" button.
3. The "File to restore" is nusrmgr.cpl
4. "Restore From" either your Windows XP CD or the I386 folder on the harddrive (if it's there); either type in the path or use the "Browse To..." button
5. "Save file in" C:\Windows\System32 - either type in the path or use the "Browse To..." button
6. Click the expand button
See if the User Accounts applet will then open.