1) buy a ME disk from ebay from around a tenner. They shouldn't really cost more than 50p.click here
2) if you haven't bought the upgrade disk yet, consider getting an OEM version of windows XP click here. This will cost you around £56, cheaper than a upgrade disk but you will need to format the hard drive to use the OEM disk. You could, if you haven't already got one, create a partition and store all your files and stuff on the partition. You will have to buy a cheap mouse also to qualify for the OEM disk.
3) if your machine is good enough, go for windows media center OEM at £75 click here. This will qualify you for an upgrade voucher to windows vista when it is released. Windows Vista is very resource hungry and you would need a reasonable computer to run it.
have you checked that your present system is good enough for XP