Also just a note, harddrives are listed in GB not GiB from the manufacturers. A 300GB harddrive has 300 x 10^9 bytes, not 300 x 2^30. So there will be discrepancy in what you bought and what Windows reports, though not one as big as you are having.
Also, I foolishly only made a 5 (something)B partition for C:, to install windows on. I forgot the OSs were so huge nowadays. Is there any way to make it bigger, using some unpartitioned free space from the drive (which I have even now)
Also, I bought a SuSE 10 distro and planned to use it, but it won't work. See thread here
Windows XP without Service Pack 1 or 2 does not support drives larger than about 128GiB or so. If you are running Windows XP you can check what SP version you have by right-clicking my computer -> properties and under the "general" tab there should be a heading "System:". One of the lines should say Service Pack 1 or Service Pack 2.
That is the providence of the manufacturer of your motherboard. So whoever made your motherboard, go to their site and get their BIOS update utility. For new boards, it will likely run form Windows, for older ones, it will require you make a DOS boot floppy.
Re: Service Pack 2
Yes MS will give it to you (at no charge) and you should get it without delay. Simply run Windows Update and it will proceede to get you to the latest updated version, which included SP2. SP2 has a great many enhancements, including to security, you want it for sure.
Re: Resizing drives
There's no MS support for this because it can cause data loss. However Partition Magic, from Powerquest, will do this.