[jyl]

More or less echoing the others.

Disconnect the old drive. Since it has precious non-backed-up data, get it isolated until you know the system is working properly.

Install the new drive. I assume you have a spare drive bay.

Insert the Win XP CD-ROM, do a fresh install of Win XP.

When the PC is working properly, shut it down and connect the old hard drive as the secondary drive, then when you have the PC running copy the data to the new drive.

Then set up a regular backup schedule. It can be as low-tech as manually copying data to a CD-ROM every week. Since this data is for your business, I would want to keep the backup off-site. No reason to let a burglary or house fire shut you down.