Hi Pauline, welcome to TS!
You've received great advice from everyone but I'd like to clarify a couple things for you.
First of all you have two drives (hard disks) in that machine. One is named Macintosh and the other Maxtor and it would be a good idea, as ABD said, to give them names. You can do that easily by going to the desktop (that picture you see behind all the open windows) and looking for their little icons, usually in the upper right hand corner of the screen.
Here is what a generic icon of a hard drive looks like in Leopard (not sure if it is different in Tiger):
[attachment=1363:4.jpg]
Note that I've given it a name which helps me identify what it is. All you need to do is click on the icon once, then slow click/hold on the name. The text will highlight and at that point allow you to type in whatever you wish. Don't use a slash in the name or other odd character to prevent any problems in the future. Dashes are OK, also don't ever start the name of anything with a period. The system will warn you but if you go ahead and do it the item will become invisible. Otherwise you can call the disk anything you wish.
Your Maxtor drive is the one you are starting up from according to the info you have posted. You can see the contents of the drives and find the available space on each easily by opening up a Finder window. You can do it as Paddy suggested by going to Finder, new window, or by double clicking either hard drive icon or by clicking on the Mac smily face on the far left of the dock (bottom of screen).
If you do the latter you will get a window that opens up to your "home directory". That is where personal preferences and documents are kept. Here is what I get when I click on my Mac face. Note that I'm using Leopard which has a slightly different looking window from the Tiger one.
[attachment=1364:7.jpg]
If I then click on the top listed drive (XL-MAX in my case), I then see the contents of that particular drive. At the very bottom of the window frame is the available space on that drive.
[attachment=1365:xl.jpg]
Now if I click on the second drive on the left I get the contents of that drive.
[attachment=1366:storage.jpg]
I actually have 3 drives and each is partitioned so I could have about 10 listed in the left side of that window but I 'unmount' the drives that I don't think I'll be needing to save the power supply and keep things cooler in the case. I can mount them as needed. When a drive is unmounted it is not available to the system or user and basically is in a 'resting' mode.
If you go into System Preferences and go to Startup Disk as ABD suggests, it will show you if there is a viable operating system on the other drive. All systems that are usable for startup will be listed in that window. Wait till the little gear stops spinning to see them all. You may also have System 9.2 on that machine for Classic applications but that is another story.
Having two drives or partitions with operating systems is great for troubleshooting problems. But it looks like Sandbox's advice may have cured your initial freezing problem. Still the space you reported having free earlier was on the non-startup drive so please check out the Maxtor drive and let us know how much space you have available there as well.
OH, and it looks like you have two DVD drives in that computer. Wow.
Don't get discouraged. So far you're doing great!