complicated problem

Here goes.
I have Avast 6 and somehow I forgot to register. When I go to the register screen it says “The product must be registered in the next 0 days”. I try to register but I receive the following error “The system cannot find the the file specified”. Ok. I try to update the program or the definitions but none of them work. I tried to unninstall the program (I have win xp, went to add/remove programs, clicked avast but nothing happened again…and the next time I visited the add/remove programs avast was no longer there…but the program is still loading on startup.
When I try to install directly avast 7 (Downloaded from the avast site) it says that another program is requesting a system reboot and can cause errors (I agree to continue as I am sure there are no programs that request this), I choose update to v7 and when it tries to finish uninstalling (v6 I presume) it just says there was an error and the installation could not be completed.
I believe it is a registry error, but don`t have many experience with registry cleaning soft and some help would be appreciated.
Thank you!

Run this according to the instructions>>http://www.avast.com/en-us/uninstall-utility , that should clean it out.

follow istructions on web http://www.avast.com/en-eu/uninstall-utility

uninstall avast 6 and 7 and reboot

then install avast 7 and reboot

register