Click 'Options' in the upper right of the Launcher screen (what you first get when starting the game) and put a checkmark next to 'Verify Files'. Login and let it go to town verifying (may take several minutes).
If you're getting 'error decompressing data', that means you're getting file corruption, either on disk, or in memory. To correct this, you will have to track down what is causing the data corruption...
Hard Disk getting ready to die.
Loose cable connections.
Overheated/Overclocked/Undervolted/bad or failing RAM
Interference from other programs (e.g. Anti-virus Utility, or a malicious Trojan)
Dust caused shorts on the Motherboard.