There have been many threads about this but my case is rather tricky. I have bought a new PC, the person who assembled the PC has installed a Windows 7 Professional Polish starter pack. The day after I bought the PC I have bought the Windows 7 Pro (OEM) English. I typed in the activation codes and it has been fully registered.
I decided that I have to change the language to English so I went to Control Panel > Clock, Language and Region settings > Changed the Format from "Polski" to "English (United States)" and it has only changed the way my date is displayed.
What can I do, there are two options I see, but I don't really like them
Use the Vistalizator~ http://www.froggie.sk/usage.html
Use the CD to format OS partition, I do not have a CD/DVD-ROM on that PC, so it would be a bit more problematic for me.