If you have a real WinME CD then you can do the upgrade version BUT it would be best to backup your data first and then format the drive during install which erases everything. Just loading XP on top of ME will have several side effects, none of them good.
If you do not have a real Win ME CD, then you could buy the OEM versionof XP from
www.newegg.com, etc. as it does not require a previous version of Windows CD.
If ME is working for you just look at Open Office or AbiWord as suggested. You don't have to spend any money at all.
By the way you cannot legally buy and download a copy of Windows. Lots of internet scams on that.