HP DC5700 for spare gaming pc?

You know you're talking about $5 for the adapter right? http://www.newegg.com/Product/Product.aspx?Item=N82E16812201040
Ebay probably has them even cheaper.

Lots of people (including myself) will recommend against this route as it can strain power supplies past what they were designed for. Surely though if the hardware is gathering dust... you don't care that much? My guess would be that 90% chance it'll work just fine. There is a very small (but distinct) chance that the PSU will catastrophically fail, taking other components with it... so there's my disclaimer... but as I say - if it's gathering dust anyway, do you care?