Trouble removing the screws

I have a "new" (Open Box) Mac Mini 2.0 ghz Intel Core 2 Duo that I bought through PowerMax. I want to upgrade the RAM, and I'm having trouble removing the four screws around the base of the inner chassis. I'm using a Phillips-head PH-00 screwdriver. Last night, I was able to get three of the screws out, but couldn't get the last one. I gave up and just put the computer back together. I'm trying again tonight, and I'm having the same problem. Am I missing something? Is there a preferred order for removing the screws or something? The screws giving me the most problem just won't come out. No matter how long I turn, they just won't move. It feels like the screwdriver has good placement, and I can feel something moving. I've watched several videos online of this procedure, and it seems like most people can just pop them right out. Any advice?