Very strange... it doesn't look like dirt to me... Dust tends to make a single even line in the direction of the scan.
Was the photo scanned vertically or horizontally and rotated?
That could be a big clue.
Also, did you change the USB cable that links it to the computer? These have a ferrite filter on them to remove noise. A standard cable doesn't.
Have you tried scanning with vuescan (
http://www.hamrick.com/)?His software doesn't use the standard TWAIN drivers that come with the scanners and is a great help in troubleshooting stuff.
If you scan with the demo and it gives OK results, you have a driver problem.
If it still looks like before, it could be a scanner problem.
What makes me think it's not dirt is that the artifacts look like colored Gaussian noise...
Could be related to the cable. Also, make sure everything is well plugged in (including the power supply).
Try them in the order that you feel like and keep us posted.