Welcome to TSG and to computing in general.
You have much to learn and you are lucky to be able to learn on a free machine! NT4 with sp6 is a good OS, but not very friendly to a novice. It is fairly stable and with your hardware, you probably won't be able to upgrade to W2000 and definately not to XP. Be sure to buy an Anti-Virus program such as Symantec's Norton AV or McAfee. Browsing is two-fold, meaning to search the local network for other machines (probably not what you need to know for a home PC) and Internet browsing. IE is Microsoft's Internet Explorer browser and is the most widely used, with Netscape a close second. Some sites work better with one or the other, but most sites work well with either.
Virtual memory is the process of the system using disk space as memory for programs that require more memory than the physical RAM. The usual formula for sizing the virtual memory is 2 1/2 time RAM (128MB RAM, approx. 300MB Virtual, also called a page file). You can look under the Help menu of explorer to search for haw to set in NT4 (sorry, I have forgotten the exact method).
To clean up the disk of unneeded files, you will need to first try removing programs that you don't need or recognize as useful. If you have questions, post back. To remove programs, click on Start > Settings > Control Panel, when the control panel window opens, click on Add-Remove Programs and scroll down the list of installed programs. Click on the ones to remove and follow the instructions. You will probably want to to then defragment the disk. Fragmentation is when the parts of a file get seperated physically on the disk and therefore take a longer time to open as the system has to find all the parts. There are free defragmentors available on the web, use google.com to search for one.
That should get you started. Have fun!