I have just beaten the Golum and Wilhem won't follow Cristof or the girl vamp. . . Any suggestions? He won't attack in battle unless I am controlling him and he stops attacking when I choose to control a different character.

Hi,
There should be a small green circle on the right side of the health/blood pool bars.
Is that green circle lit ?
That's how you can order the other chars to follow you or to stay put, so maybe you clicked on it and turned it off by mistake ?
Check the attached pic and look at the bottom for the yellow arrow.

Have you tried changing his stance ? Is it on 'Agressive' ?
If you take control of Wilhelm, does Christof follows you around ? If he does, maybe try leaving the area while controlling Wilhelm, and at that point try switching back to Christof.

Does he follow you when you click on the "regroup" icon (four arrows on the bottom left corner of the screen)?
If he doesn't then it most probably is some strange bug.
In that case we can try generic "solution to everything trick": are you using Vista or Windows 7?
If yes - then make sure that you install and run the game "as Administrator". This is just a guess, but it helps in so many cases that it's worth a try.
Edit: Stance shouldn't have any effect on coterie members following the selected character.
And on a side note: aggressive stance should be renamed to suicidal :P

Thanks for your suggestions guys, however I am not experiencing any fix. I've tried toggling everything: stance, pressing the regroup button, changing characters, the green "follow" buttons. I am also running this program as an admin since I was experiencing save game issues as well. Furthermore, Wilhem will not follow the group no matter who is leading; he won't follow Cristof, Serena, or Erik now that I have him.
In addition to being really annoyed that he won't follow, unless I am playing him directly, Wilhem will not fight during battle - he just stands there like a useless cretin.
I am running Win 7. . . *cry*

Looks like a corrupt save file I'm afraid.
I'm sorry for another generic question but have you tried reinstalling?
Maybe you could upload your save file, so we could determine if it really is a corrupt file or or a game problem?

So should I just copy the save files and then put them in the new save folder once I reinstall?

I use WinXP so don't really know how Win 7 handles saves, but I assume it would work - copy the files to the safe place, reinstall and then copy the files to the right folder. Where the files are stored appears to differ on the compatibility settings.
But I think that we should check if the file is corrupt first.
This is what you do (I just checked and this trick works): compress one save folder (like SP1 for example) into a file like SP1.rar. Then change the extension to SP1.jpg and attach as image to your post.

Yeah, backup your saves (copy them someplace) and once you reinstall the game just drop the saves in the save folder. But if it is the save file that is corrupt, i don't know how much good it will do you. Give it a shot anyway.
Or if you have saves before the bug first crept up, maybe try to reload one of those and see what happens.

That's why I suggested posting the safe file here - a 1.67 MB safe file can be compressed to 145 KB, so it can be attached.
Edit: By safe file I actually mean save folder corresponding to specific slot. There are files responsible for different locations inside.

Ok - attached is the most recent save file folder, SP8 in .jpg format.

Nope, your save is buggered. Not only does he not follow the cotterie, enemies act as if he's not there. Take him alone beyond that door where the save starts, put him dead smack in the middle of the two lasombra ghouls around the corner (see att. pic), and they don't even acknowledge he's there...

As Namur said - your save file is corrupt. That means there is no way to solve this.
But this isn't as bad as it seems - enemies won't attack him until he attacks first ;) It's like a constant Cloak of Shadows.
There may still be one "solution" left. I have an old save file from Stephansdom 3. I can use cheats and provide a similar safe file - the same inventory but different disciplines (there seems to be no way to decrease discipline levels and only increase is possible).

Thanks a lot guys for your time. Would you mind posting the save file from Stephansdom 3? Thanks again. . .