+1, Drilling a rim is risky business. There's the problem of getting a properly drilled hole, filing the hole so that it doesn't cut valve stems, and then, if it's a narrow rim, you have to be concerned about rim cracking where the hole was drilled out. I know others have drilled out rims successfully, but I wouldn't. sonatageek has the correct solution. When I first encountered Presta valves, I was all set up for Schrader valves just like you. I went the route of the adapter. It was no big deal. Sometimes I carry that adaptor today just in case I flat and I need help from someone only set up for Schrader.
And, changing to the adapter is A LOT easier than drilling out a rim, and costs a heckofa lot less than buying another rim.