how to put key back on keyboard?...

There are a couple of small pins that snap in to the key assembly. If they are broken, you can try to glue the key top on, or buy a replacement key or entire keyboard from e-Bay. The single keys can be expensive vs. the entire keyboard.

 

Dimension 9100, Dual-Boot Win XP / Vista Home Premium, 3.0 GHz P4, 3 GB DDR2 533 MHz RAM, 160 GB SATA II Samsung (XP), 300 GB SATA II Seagate (Vista), 250 GB SimpleTech USB (WD Drive), Nvidia Go 6800 (425/825 MHz - XP, 400/800 MHz - Vista, Vista Driver - 163.75), Dell 1901 UltraSharp FP

Inspiron E1705, Win Vista Premium, T7200 Core 2 Duo (4MB, 2.0 GHz 667MHz), 2 GB DDR2 677 MHz RAM, 120 GB Samsung HD, Nvidia Go 7900 GS - 156.69 Driver, 17” Sharp UltraSharp TrueLife Wide-Screen WUXGA