You can enter the Pro License and provided you are on-line it should convert to the Pro version. Though since you are on win98SE I would suggest you download the Pro Trial version, just in case so you have it available to do a clean reinstall.
If you need to do that:
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
2. run the avast! Uninstall Utility, reboot.
3. install the latest version, reboot.