I thought I might put up something for others to go by when asking for help.
First off it's always easier to help someone with problem when your explanation of what's happening goes beyond "it's broke". Give as much detail as you can as to the nature of the problem. Report any error messages, what you were doing at the time the problem occurs and what kind of result does the problem create. IE crashing to desktop or locking up.
Here are some other questions that would be helpful to have answered:
1- With ArmA specfically what version are you running?*
* This includes what revision level (which patch level your at) and what country version IE, Czech, German or other
2- What are your system specs?**
3- Are you using up to date drivers for your motherboard chipset, sound, and video card?
4- Have you added or removed any hardware or software between the time everything was fine to the time that this problem occurred?
5- What steps, if any, have you taken to correct the situation?
6- Check if your issue is a bug. For this use the official wiki buglist.
** IE motherboard make and model, type of video card, how much video memory, how much system ram, powersupply size, HD size and it's free space, what sound card do you have, and what operating system you are using?
If you can answer those questions when posting your problem that would certainly be helpful.
[EDIT] by Foxhound, added correct link to buglist