I think you may be referring to DOS:
Boot into dos.
1 attrib -a -r -s -h system.dat
2 attrib -a -r -s -h user.dat
3 ren system.dat system.aaa
4 ren user.dat user.aaa
5 attrib -a -r -s -h system.da0
6 attrib -a -r -s -h user.da0
7 ren system.da0 system.dat
8 ren user.da0 user.dat
9 attrib +a +r +s +h system.dat
10 attrib +a +r +s +h user.dat
I think this is it -
steps 1-2: unhide the registry files.
steps 3-4: backup the registry files - just in case
steps 5-6: unhide the system's backup registry files
steps 7-8: rename the system backup registry files to the original
steps 9-10: put everything back again.
After each line press the <enter/return> key. DONT type in the numbers - they are just for a guide.
It is possible this is wrong, as it has been about a year since I needed to do this.
This is for Windows 95/98/98SE/ME - not sure if it would work for anything else.
Good luck