We've had this problem. Hubby's not a writer but a programmer. We have separate computers - we have 4 total in the house. Somehow, though, MY time has been too easily taken away in the past. I had a meltdown about it a couple of weeks ago, and it's better.
For me, it's that I'm home during the day, which means that I take care of most of the household administration/cleaning. Over time, DH has taken care of less and less. Now, I've insisted that he takes the kids and not bother me at all when I'm working. When he starts to bug me, I've starting sighing really loudly, and he usually takes the hint.
Still, I'll go upstairs after the kids are in bed to find that dinner dishes didn't get cleared, toys didn't get put away, and then I have to face doing them late when I want to go to bed or dealing with a messy house the next day. So, really I have no advice, but I understand.
My husband also doesn't use the time he does have. He'll stay up watching a movie or reading, which is fine, but then he wants to use my writing time because he "never has time to do anything." When I ask about the 2 hours he was reading the night before, I'm suddenly the bad guy.