I'd suggest you first uninstall the existing driver then reinstall it using the exact instructions below:
Click start, settings, printers, right click the printer icon listed in the box, left click delete.
Now
1.Double-click Add Printer.
2.Click Next.
3.Choose HP from the Manufacturer list in the left window.
4.Choose model HP Deskjet 500C from the printer list in the right window.
5.Click NEXT
6.Select Port (usually LPT1) then click Next.
7.Select Yes if this printer is the default printer, or select No if this printer will only handle selected print jobs. Click Next to continue.
8.Select No to skip the test page then click Next.
9.Click Finish to complete the installation.
10.If asked for a Windows disk, insert the appropriate Windows 98 or PC Recovery disk and follow the instructions on the screen. Once the driver has been installed, it will be listed in the Printers window (Start, Settings, and Printers).
11.Close the Printers window and try printing.
If you have trouble with garbage printouts after installing this driver, check your lpt bios settings and make sure it's set to manufacturers suggested settings. eg: spp, ecp, etc.
If you still have the same problem after reinstalling this driver, then you most likely will need to take it to a repair shop.