I had success getting the HP Color LaserJet 3550n (the 3550 family ) printer to install in Win7. That printer was the only thing that did not fire up with Win7. In the Devices and Printers window, I tried troubleshooting, but it was a problem with the DOT4USB drivers. Similarly to others, I was successful by using the following course of action:
1) Went to HP and downloaded the Vista 32-bit drivers for the 3550, unpacking them at C:\HP CLJ3550\ as suggested by the driver file.
2) In Devices & Printers, did <Right-Click> <HP 3550> <Properties> <Hardware tab>
3) I selected the first "Device Function" <DOT4USB> then did <Properties> <Change Settings> <Driver tab> <Uninstall>.
4) I then selected the second and third Function (which were <HP Color LaserJet 3550> and <IEEE 1284.4 compatible printer> and did the the <Properties> ... <Uninstall> for each.
5) I disconnected the USB to the printer, went back to Devices & Printers, and selected <Add a Printer>.
6) I selected <Add a local printer> <Use existing port DOT4_001> <Next> <Have Disk> <C:\HP CLJ3550> <OK> and installed the printer.
7) It would not work unless I "let" Windows make it the Default Printer
8) After it was finished, I did <Right-Click> on my preferred printer and selected <Set as default printer>, and everything worked.