Installing a new OS on Power Edge 840

No client OS is supported on any Dell server.  That doesn't mean it won't work, but it "may" not, and it is up to you to find drivers and troubleshoot any issues during and after the install, as Dell provides neither.  That said, it "should" work, and any drivers you might need, you can usually use the 2008R2 drivers (unless you are trying to install W7x86).  Which storage/RAID controller are you using?  Also, you should realize that the onboard video will NOT be up to par with any desktop computer, and outfitting a server with a video card is a very hit/miss adventure.  Why do you want to install Windows 7 on it?

We have downsized from the time we first purchased the Power Edge 840 server about 10 years ago. We are still running the original OS which is XP Professional 2003. The only software we currently use on the server is Quickbooks 2013 serving just two (wireless) client computers. Each client has Windows 7 Professional and I assumed that having the same OS on the server would be more efficient particularly since we occasionally have network issues with our printers. We are also considering purchasing another Power edge 840 with a faster processor and more memory. If we did this what would be the best OS we should install since the machine we are considering does not include an operating system. Or, perhaps you could recommend a different solution that would better suit our needs. Any advise would be greatly appreciated.

The PowerEdge 840 is not 10 years old.  I was probably released around 2006-2007.  XP (available starting in 2001) would not have been the OS installed from Dell, but would have to have been installed after receipt of the server by in-house/local personnel.

Server 2008R2 would be your best bet to run on the 840 now.  I'm not sure what kind of "network issues" you are having with your printers that you think is associated with the server OS, but there should be no issue with 2008R2 in that respect.