This symptom is due to the specifications of Windows XP.
The Start menu has the function of showing the programs that the user uses frequently automatically.
If the user deletes a program from the list, the number of use of the deleted program becomes zero. When the program is used again, the number of use is counted and shown in the list again.
For more information, please refer to the following solution in Microsoft's technical support website.
Article ID: Q282462
Last Reviewed: August 4, 2001
Title: Programs Removed From the Most Frequently Used Programs List Can Reappear.