How do i open an file with extension .sit.hqx.hqx ?

Hi,

i'm new to mac and have downloaded a few files, alot of them end with the 'hqx' extension (.sea.hqx, .sit.hqx, etc). When i double click on the file a window pops up asking me to choose an application to open the file, but i can't see any kind of installer app.

i've had a bit of a look on these pages and can't find any postings regarding this, could someone let me in on the secret please?

Thanks,
Sean