It's been a long time since I used an XP machine, but try this, if it doesn't work I'll have another think...
Right click a video and in the pop up list hover the cursor over >Open with<
Here a pop up will list all the players that are on your system, ignore them, even if WMP is listed. Instead Left click >choose default program<
Select >use the standard Windows Program< then OK
In the pop up Left click the box that says...
>always use the selected program to open this kind of file<
Then either select the player you want to be your default or if it isn't in the list browse for it.