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Depending on the type of program, you would FIRST need to INSTALL the program on the WinXP computer before you could actually RUN the program.
If you had the INSTALL files on the Win98 hard drive, then, in theory, you could RUN the install package FROM the Win98 harddrive and have it INSTALL on the WinXP harddrive.
However, most programs are written specifically for an OS. SO a program written for Win98, wont run on a WinXP OS. Make Sense?