I don't think any company is going to cover milk damage. Warranties are intended for a product that fails due to a lapse in design or build quality, not for damage caused by the user. They would almost certainly reject the warranty request when they open it and find liquid damage and the smell of rancid milk. : P
And one shouldn't open a PSU unless they are skilled with electronics, since they contain capacitors that could potentially produce a fatal shock even after being unplugged for some time. If milk only got into the PSU, then you should replace the PSU with a new one, and see if the rest of the system is in proper working order. Don't try powering the system up with a milk-filled PSU, and you should keep the power supply unplugged.
What are the specs of your system? A new, quality PSU shouldn't be too terribly expensive, probably not much more than $50 USD for one that will probably be better than what the system came with.