As a test, place the site in compatibility view in IE10 to see if that may help if it's an issue with the site not working properly in IE10 versus an earlier version of IE.
Indeed - if it worked in XP under IE8, did the same site work in Win7 under IE8? If so, did it work in IE9 and IE10? IE8 to IE10 is a pretty huge jump, and while it's possible this has been tested, it's not documented here in the thread so I don't know when it worked, and when it didn't (version-wise). IE10 seems to be a bit more picky about form fill on https sites versus http too, in my experience.