Well firstly, you can easily run linux on your computer. Though I guess it will be a bit slow since it uses KDE which is a big resource hog. To run it nice and smoothly, you may have to use some windows managers. They are a unknown concept to users of MS Windows and a bit tricky to use in the beginning, once you get used to it, your PC will feel lot faster than it would feel under windows. You can't install LInux and Windows in same partition. You will have to give separate partitions. Don't worry about formatting part. Just leave empty partition on your computer to install linux. If you do decide to remove linux in future you will again have to format the disk to use it for Windows,
Don't bother with fdisk command, unless the data on your PC isn't too important to you. Incorrectly done fdisk can lead to data loss. Leave the partitioning part for Disk Drake (i guess) used by mandrake.
You can do almost everything on linux that you can on windows thorough linux versions of the softwares eg. firefox for linux which is the same as windows, or some alternative softwares like NVu for web designing instead of Dreamweaver. One place that linux sadly lacks is gaming. But, I don't think that you'd be interested in gaming on that computer which the specs it has. You have absolutely no need for anti-virus in linux.
You can search
google linux for any query on linux. Or you can always post here.
Have a look at this thread for your software needs
http://www.thinkdigit.com/forum/viewtopic.php?t=15797