Alright. Here are some things to try out. DO THEM IN ORDER! It may take you a while to complete this stuff, but it will help you with the time you invest.
1> Run "Windows Update" from the Start Button Menu and install all the updates except the Language Packs. Since you reloaded the PC Yesterday, all the updates need to be run to get the PC up-to-date and current again. This is very important! Some updates will require several reboots of your computer. Go back each time it reboots and make sure there are no more updates to run. Once they are all installed, move onto the next step. DO NOT JUMP AHEAD!
2> Update your graphics card drivers to the latest version... They can be found on
www.nvidia.com. Your PC is an HP Pavilion Media Center m8120n Desktop PC so your video card shold be a GeForce 7350LE. The drivers can be found at the bottom of our BOOM! Home Page
3> Install the latest version of DirectX once all your drivers are updated. This will help a lot. Microsoft's DirectX installer can be found here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=2DA43D38-DB71-4C1B-BC6A-9B6652CD92A3&displaylang=enUpdate us after you do this stuff and be very clear as to what's going on when you post your problems so we can be of help to you.
Good Luck and hang in there. Lots of people are struggling with Portal, so with a little patience, we will get it working for you.
Mav