I have Intel Pentium3 motherboard model 845GVSR with 40 GB Hard Drive, DVD/CD ROM and 4 USB slots but no Floppy Drive.
My PC was working fine until I deleted the contents of hard drive accidentally using trial version of killer software.
I tried to install Linux by making a boot-able CD, but it did not work.
Then i tried to make a boot-able USB using Universal Notebook Installer, it did not work either. I just get the Error " Error loading operating system.
Then I tried a free software from net by the name James Format Tools - DOS on USB. Using this computer did boot but DOS did not install and I got the error message "invalid drive specification".
I understand that now I will have to write boot sector afresh and will have to Format the hard drive,, but how to do this as all my attempts to get to the hard drive failed
Thanks in advance for help

1) Check the BIOS. Make sure that it is set to boot from the CD-Rom first, before the Hard Drive.

2) Did you burn the ISO properly? Does the disk work in another computer? Quite often newbies will just copy the ISO to a CD, rather than "burning the iso" properly. If you have any doubt look at https://help.ubuntu.com/community/BurningIsoHowto

3) SOME distros have the ability to partition/format automatically, some do NOT. Which version of Linux are you trying?

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You need to undelete the files. Get testdisk.

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As suggested, check that you burnt it as an image.
An old PIII isn't likely to have boot from USB. And I have seen BIOSs that require boot loader code in the MBR of the (first) hard disk. The only way I was able to fix those machines was to remove the disk and write the MBR from another machine, then put it back. Then they would boot the CD.
As for testdisk, that works by scanning the disk - any sort of "killer" code will have delete/randomized all of the disk. And the OP can't boot CDs as yet anyway ...

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It might be that your system never did offer a way to boot to cd's. It also may boot to "pressed" cd's but not burned one's or some burned ones. We would need to know if it ever booted to a burned cd or a store bought cd.

Older might boot to special cd's made for older systems too. It is a syslinux or isolinux deal. A puppy cd burned at slowest speed on a black cd may have a chance of working.

Since you would have trouble deleting the contents of C: under windows to some extent I'd wonder what "killer software" is?

Last way to boot would be to a floppy that taught the computer enough to boot to a cd or you may be able to network boot.

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Ty all for the help
I was able to boot and install the software following the instructions at post no. 2