When you restored the computer did you reinstall the drivers for the sound card or onboard sound? Go into Device Manager, then click the ( + ) sign next to, "Sound, Video and game controllers". If you see a yellow exclamation point an issue is present. If your sound card or the name of your onboard sound (comes on the motherboard) is not present, look at the bottom of the list for generic or unknown hardware or something of that nature. When a hardware component is detected but not recognized by name it is put into a temporary category. Once the issue is corrected, which usually is proper installation of its related drivers, then it is recognized by name and added to the appropriate section, which in this instance would be the one I just listed.
If nothing is wrong with your drivers, I still recommend you download the most current version and install them anyway. Sometimes drivers can become corrupt and cause issues without showing any issue in Device Manager. Please be sure to look in Programs and Features or Add/Remove Programs for the sound drivers and then remove them if listed. If not listed just download and install.
I know what I say next sounds silly, but check your connections. Sometimes it is as simple as that, and I know I have overlooked loose connections because it IS such a small thing. Also, check on your speakers, especially the bass if you have one, for a volume control. I had that happen too, mine somehow was turned Off on my Bass and it stopped all sound internally.
If none of this works ask your dad if he fiddled around in the bios. If so, go in and ensure that the sound card is enabled. Often motherboards have sections which enable or disable certain hardware components. If that isn't the case, you may want to ask in a technical assistance online forum, and I will provide 2 links, one for a third party forum and one to a Microsoft TechNet forum, both are good for this type of assistance and are free.
http://www.bleepingcomputer.com
http://www.microsoft.com/communities/newsgroups/en-us/default.aspx?dg=microsoft.public.windows.vista.hardware_devices&cat=en_US_9CA88DDB-D18D-FA0E-A366-6E527B0FBA67&lang=en&cr=US
First search the forum for your issue because often others have had the same or similar issue and you may find a solution without having to ask your own question. They are simple to search and you can do this without registering or needing to logon with your Passport credentials, (Microsoft forum). If you do need to ask a question please be sure to provide basic system information such as make and model, processor, RAM, sound card, current driver versions, anything leading up to the issue, any error messages, and the troubleshooting steps you have taken to date and the outcome.
The Microsoft forum requires a Passport or credentials, if you need to ask your own question. If you have a Hotmail or Windows Live Mail account that address and your password is your Passport. If you do not have one of these accounts you do not need to open one, any valid address and the password you use to access it can be attached as your credentials. It is simple and quick to get your Passport. Simply click the Login button in the upper right hand corner of the page, and when taken to the Login page, click on the link to get your Passport credentials. Follow the steps and then activate within your email account by clicking on the link in the email that is sent to you.
Good luck and please have a Happy New Year.