One of the very frustrating things that comes with this computer is when you have to reformat the machine and install a fresh copy of windows. Here are couple things that I found fixed the very common but almost unsolved problem with these laptops. These fixes are also for the C400 and other similar compaq and HP models.
*Compaq Presario C300 Sound Card Issue and Fix - Most everyone has this issue when installing a fresh copy of windows on their C300. I was frustrated with it myself until I done a little trial and error until it was fixed. Basically its pretty easy. When you install a fresh OS install your video driver along with all of your other drivers for the laptop. When it comes to the sound driver for the 3D Sound Blaster Pro you will need to MAKE SURE you have windows service pack 3 installed on your computer if you are running XP. Once this is installed you will want to go to www.compaq.com and download the softpack driver called the "Microsoft Universal Audio Architecture (UAA) Bus Driver for High Definition Audio" which is soft pack sp32646. Once installed restart and your sound card should be recognized and working!
*Compaq Presario C300 HDD not found issue- Again this happens sometimes when doing a fresh install on windows after the HDD has been partitioned and reformatted. It is another simple fix and all you have to do is go into Bios, for to the System Configuration tab, scroll down to "SATA Native Support" and disable it. Safe your new setting and restart the computer. As long as your hard drive is not throwing the error because of a hardware issue this should solve the problem.
*Compaq Presario C300 no display, power button flashing- This another common problem with these computers that's all over the internet without a solution posted that I can find. After inspecting the motherboard and giving it the OK I looked the computer over and figured as a last shot I would pull the button cell battery from the motherboard and reset the bios/cmos. Upon doing this I assembled the laptop and it came on the very first time and worked great for the customer ever since. If this doesn't work for you remember that the battery may also be dead. Before trashing the computer or ordering expensive parts go out and buy a CMOS battery and see if that does the trick.