I have it working now, I added start to the beginning of each line.

start C:\miner\mining_proxy.exe -o stratum.bitcoin.cz -p 3333 -gp 8332

start C:\miner\mining_proxy.exe -o eu-stratum.btcguild.com -p 3333 -gp 8333

I'm not sure if that's an obvious thing or not.

Thanks for your help with this.

I have noticed that the efficiency of my blade set to BTCguild is around 93% now when it used to be around 98-100% on Slush. Is that to be expected? My two other blades on Slush are still around 99%.