Cirroco,
Windows XP works fine with Avast and there is no need for "workarounds"
There apparently is something wrong with the installation.
Firstly, I would run the Avast Repair which you can access by going to Control Panel, Add/Remove and clicking on Avast. Use the option to Repair.
Reboot computer and try again.
Please let me know exactly what the error messages state and include any pathway shown.
Thanks.