I'm assuming that you have a set of Windows 95 installation floppies - if not don't even think of doing the following.
Download a boot disk click here
Boot with the boot disk in the drive; this should take you to the A:> prompt.
and hit Enter.
This will wipe the hard drive.
Then reboot with Disk 1 of the Windows 95 disks in the drive and follow the on-screen prompts.