su command gives you super user (root) access. i.e. you can do anytihng you want ot the system and it won't stop you. Yes I've completely screwed up my system before. But its necessary to make changes to your system.
If you have an Nvidia video card you will need to dl the nvidia linux drivers. I couldn't log into any sort of graphical mode until I downloaded them. I assume you have a dual boot system set up.
Go to nvidias web site, downlaod the drivers, burn the file on to a cd. Boot into linux copy the file from the cd. You will have to mount the cdrom....I think 'mount /dev/cdrom' should do it, but I'm a little shady on that. Install the drivers according to the instructions given on the website.
If all goes well 'startx' should get you into the graphical mode.
Hope I helped and if not....well my mom smoked crack when she was pregnant with me so I have an excuse...