Computer abruptly stopped connecting to internet on start up

I just got a new Dell Vostro 430 Mini Tower. It was working great, I went on vacation for a week, and when I came back and started up the computer, it was not connecting to the internet by ethernet or wireless. There was nothing wrong with the internet connection in the office, I restarted the computer, and it did the exact same thing. One of the guys managed to use the troubleshooter option to get it working, and it all seemed fixed.

Yesterday, I came back to the office, turned on my computer, and it wasn't connecting again. I tried using the troubleshooter, but it couldn't figure out what was wrong and didn't give me any useful options. Finally, I disabled my connection to the wireless and additional networks, then clicked the X signifying that the computer wasn't connecting to the internet, which caused it to reconnect to the wireless network (but not the other networks). Then the internet connection was fine for the rest of the day.

I was hoping that solved it, but I had to do the same thing again this morning. I don't know why it's suddenly doing this, and it is taking about 5-10 minutes each morning to get the computer working, which is really slowing me down (and is part of the reason we got a new computer in the first place). Anyone know how to solve this?