If you want Windows to stop being in English, and start being in Polish, follow these steps:
Note, that you will need to be the administrator on the PC.
Go here and download the Windows XP Service Pack 3 installer in Polish, but do NOT run it (yet): http://www.microsoft.com/pl-pl/download/details.aspx?id=10767
Open the registry editor by opening the "Run..."-box (Start -> Run - or just press the Windows key and 'r' simultaneously) and then typing 'regedit' (without the quotation marks).
In the left panel, navigate to HKEY_Local_Machine --> SYSTEM --> ControlSet001 --> Control --> NIs --> Language
In the left panel, right-click "Language" and select Export. Save the .reg file somewhere you will remember the location of - this file is important, as it serves as your backup, in case something gets messed up!
After highlighting the 'Language' option in the left panel, you should see a whole bunch of four-symbol codes in the right panel. Scrolling to the very bottom of the right panel, should make you see two options,
InstallLanguage - right click these two values, and select "Modify".
Now type in the numerical value for the Polish version of Windows - 0415. Now do the same for the other value (depending on which one you chose first).
Now restart your PC (it will not work if you do not restart the PC).
After restarting the PC, run the Service Pack 3 installer, you downloaded earlier.
Upon successful installation of Service Pack 3, restart your PC (once more).
After the restart, Windows should now (largely) be in Polish! There may be some features, that are still in English, but for the large parts Windows should now be in Polish.
To see a full list of the language-codes, as well as the guide I "stole" this from, see: http://jekkilekki.wordpress.com/2009/01/05/changing-os-languages-in-windows-xp/