Download the Valve Hammer editor for HL1 from collective.valve-erc.com instead. Get the Zoners (ZHLT) compile tools also.
You'll have to go to
http://www.zhlt.info for the zhlt tools as valve-erc's link to the tools is no longer valid.
I don't know about deleting your old HL1. Think about keeping it. What happens when steam doesn't work? Hmm? I kept my old HL1 version for playing bots and testing HL1 maps. It's faster, etc.