The countdown value is contained in BOOT.INI on your "active" partition. But it's a "hidden" file (attribute H) and not normally shown in Windows Explorer.
You have to change your Tools -> Folder Options -> View tab, to check the "show hidden files, folders, and drives" in order to see it.
However it is also a "system" file (attribute S) and not normally eligible for editing. So you have to use some program capable of altering attributes to temporarily turn off that H attribute. You could even turn off both S and H at the same time, if you didn't want to change the "show hidden files" folder View option.
Anyway, if you eventually get it open with Notepad you can edit it, and save it. Then you can restore the S and H attributes, and you're in business.
Just don't screw up the editing. Probably a good idea to create a restore point before you do this, just in case.