So you may have updated you client to Minecraft 1.3 No biggy!
No.
You may need to rename it to minecraft.jar or just minecraft
Press the windows button in the bottom left hand corner, type in %appdata% and press enter. Then in the folder that opens find .minecraft
Next, (make sure you Minecraft is closed) click on the bin folder and delete the current minecraft.jar (sometimes just named minecraft)!
Then paste in the downloaded jar I gave you above!
No.
You may need to rename it to minecraft.jar or just minecraft
Press the windows button in the bottom left hand corner, type in %appdata% and press enter. Then in the folder that opens find .minecraft
Next, (make sure you Minecraft is closed) click on the bin folder and delete the current minecraft.jar (sometimes just named minecraft)!
Then paste in the downloaded jar I gave you above!