First make two accounts on XP. The first will be by default a administrator. Use this account to administer your system only. Make your second account a limited user. Use this account for all your daily task. If you are logged in as a administrator when you encounter a virus, trojan, or adware, it will have full system write permissions just like the administrator that you are logged in as. If you encounter a virus, trojan, or adware while logged in as a limited user, it will not have system wide write permission, and will not be able to make system wide changes.
Secondly, VirtualBox has a option to take a snapshot of your VM. If Parallels has this feature also, get XP how you like it, take a snapshot of your VM, if you run into trouble, you can revert to your snapshot.