If you can restore your backup if you can't or you didn't make one you have a problem heres what to do: 

1)Boot from a DOS boot disk 

2) If you can access your hardrive from DOS copy anything important onto disks 

3) Go to c:\windows\command and copy the file FORMAT.EXE and EDIT.COM to your boot disk along with your cdrom driver and MSCDEX.EXE if necessary. 

4) Re-boot and type FORMAT x: (x= your Hard drive)

5) If you have the Windows 95/98 upgrade your going to have to install your old OS then install your upgrade. 

6) If you have the full version of your OS  install it (or windows 95 type d:\ (d= your CD rom letter/ or floppy drive letter if you have Win95 on disks) then setup. 

7) After re-installing you can restore lost data from your backups. 

8) Windows should be back!