There are two ways to do it, the best way is to get a program to zero fill the drive, I use Seagate DiscWizard Starter Edition available from here :
http://www.seagate.com/support/disc/drivers/discwiz.html
Its free and will completely erase all the data on the drive by writing a zero to each part of the drive.
The other way is to get a 98 bootdisk from :
http://www.bootdisk.com/bootdisk.htmUnless you already have one.
This is probably good enough for what you need, and with this you set the bios to boot from the floppy first, stick the floppy in and start the computer.
It will ask if you want to start the pc with or without cd rom support.
At this stage select without cd rom support.
When it has finished doing its stuff you will see the A prompt.
A:\>
Type format c: and press enter, it will say warning this will delete all data on the partition do you wish to continue, say yes and it will wipe the partition on that drive.
I assume if its an old pc running 95 it will only have the one partition.
If it does have more than one partition you will need to type fdisk at the A:\> and delete the partitions first, then format as above