Today whilst working on a clients ThinkPad T60, I experienced the same issue.
I had re-installed the NB to windows 7 32bit, The automatic search in 'device manager' found every other driver besides this underlined "Unknown device" >> details >> ACPI\ATM1200"
I then proceeded to download every obvious driver I felt was required, through the appropriate website >> http://preview.tinyurl.com/3apstxu
I can't recall the poster's name, but I did see a post relating to " TPM(Trusted Platform Module)" as being this 'UNKNOWN driver'. And this is exactly what fixed the issue.
I downloaded this XP,Vista Version of the driver TPM, as I didn't see one for win7 >> http://bit.ly/nP7vdT
When you run the .exe It just looks as if it installs too quickly despite It's small size.
I then tried to install it a different way by opening D
anager >> Right click on the 'unknown driver' >> Select 'Update Driver Software' >> then 'Browse my computer for Driver Software' >> In the following ('Search for driver software in this location' Textbox) I replaced the current directory path with 'C:' >> Kept 'include subfolders' checked and proceeded to Click NEXT. Voila
I just by default run a search from the root of the primary partition C:
Good Practise, and never takes long anyways.
Hope this finds you well !
If you don't at first succeed. Stop. Relax. And now read my post properly