I went into Kcontrol and selected a new theme. now there are no buttons to close windows or minimize them. the taskbar dissapears when I run a program and doesn't come back. the keyboard doesn't work either (I'm typing on a different computer). I'm NOT having a good first impression with linux, however it's only been a day...
should i reinstall the os?

is this something that happens every time you load kde (i'm assuming your using kde). and is your keyboard not working? or is kde just not responding. if kde is locked you might be able to restart it. pressing ctrl+alt+backspace should kill the desktop and return you to a command prompt. (you can also access a command prompt with ctrl+alt+(function key), ctrl+alt+f7 to return to kde.)
i'm not using mandriva right now, so i don't know what themes are there, but i know that kde is very customizable and you can change almost everything, including where and how the taskbar is displayed (it can be set to hide) and whether the title bar buttons appear or not. look at some of the specific appearance settings. that might be all it is.
jacob