I have installed an HP DeskJet 4620 driver on a win 7 machine. All works perfectly for several days, and than printing is not longer possible. Instead I get the message: "Unable to communicate with printer".

This happened on every Win 7 PC I tried, and none of the HP/MS sites contain any relevant info...

(Posting this so that the answer appears online, as I did solve it after much work)

Solution: It appears that HP installation puts a unique "port" called "HP Network re-discovery". It stops working after some time (possibly after the first time the printer/pc enter sleep mode).

BUT, the standard MS TCP port works just fine.

So:

1. Go to "Printers"
2. Right click Printer
3. Click "Printer properties" and then "Printer" or "Fax" (for both - do all this twice)
4. Click "Add Port..."
5. Select "Standard TCP Port"
6. Fill in details
7. Move printer to use the new port by un-checking the old one and checking the new one

Happy printing.