Is HP Printer #842C not usable with Windows 7?

To some degree they are correct. Your printer is NOT Windows 7 compatible. However that does not mean that some of the functions may not work. According to HP it can be made to partially work with W7. However for the price of new ink cartridges for this old printer which you may or may not ever get to work correctly, you can buy a brand new printer that will work with W7 and be superior to the printer you now have. From my experience with old printers this may very well be your best solution but it is your choice. I personally would just buy a new pinter. Here is what HP say's about this issue: http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01874792&lc=en&dlc=pt&cc=pt&lang=pt&product=60426

The person at Best Buy gave you incorrect information.

See the manufacturer's web-site:

http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?product=60426

and click on the "Microsoft Windows 7 (32-bit)"
or click on the "Microsoft Windows 7 (64-bit)"
hyperlinks, and read how your printer is supported.