The GW2.exe file that you run is both the downloader and launcher.
GW2 is pretty simplistic in it's design, being that it only uses a few files to run/maintain.
Main Game folder:
Gw2.exe
Gw2.dat
Gw2.tmp
And User Folder/My Documents/Guild Wars 2:
Music
Local.dat
ArenaNet.txt
Here is my suggestion to you and anyone else during these beta phases...
Create a folder where you are gonna permanently install GW2 (C:\Guild Wars 2 or C:\Program Files\Guild Wars 2 etc etc...)
Move your GW2.exe file in there and run it.
(Go ahead and make a shorcut to it on your desktop, start bar, or start menu.)
When the game is done downloading and run for the first time, you will have the other files in there GW2.dat, GW2.tmp
The GW2.dat file is extremely large because this is the game data that was downloaded.
Create a copy of GW2.dat and store it anywhere for backup. (Even in same folder if you like)
Back this file up after every major update.
At this point, after running the game it should create the other files in the user folder.
You can choose to also backup the Local.dat file, but it is most likely not necessary.
You can largely ignore this folder.
Now, anytime you run the client and it tries to download again the whole game again, close the client and check your GW2.dat file.
If the size is small now or different then your backup, because it restarted download, then just delete it and replace it with your copy of GW2.dat.
This should prevent you from having to download the full game data file again and save you a ton of time.
While I have never ran into this problem myself, I always back it up to be safe.
I had a buddy that had this happen to him during the stress test day and he missed most of the beta because he had to redownload. Thankfully he has fast internet.
So while I can't personally gaurantee that this will work 100%, there have been others on the official forum that have had this spare them some grief.
Best of Luck and enjoy the beta.
Edited by CrimsonReLLiK, 01 June 2012 - 12:00 PM.