Disk utilities provide a 'write zeros to the drive' which
scrubs all data off the disk.

Would the store-bought reinstall guarantee to fix your problem
and save your data and provide you with new software (XP) ?

One caution with dealing with shops: only get work done by the
job, not by the hour. Installing a windows operating system
takes about an hour. $80/hr is good wages for someone or money
saved by doing it yourself.

That's my tips as a former employeee of a computer store.