Dell inspiron N5050 Made in china , bought it 3 years ago, and no i dont think of adding memory to it since i use it for important work only , no other use for it .
Well, aside from why it's "freezing" regularly, even when it's operating properly it's apparently also not performing adequately to meet your important work needs, which is why you're asking for ideas and suggestions.
So, the N5050 has two memory slots, and the absolute bare minimum memory that the machine is configured with is 1GB... which is your machine's story. As I said, 1GB is really inadequate for "decent" performance with 4GB really being the minimum Win7 system spec in my opinion. Since you are running 32-bit Windows 4GB is actually the maximum your system can support anyway. In theory, running 64-bit Windows on the machine could support up to 8GB max (as 2x4GB) of PC3-1333 memory.
However GREATLY IMPROVED PERFORMANCE (at a modest cost) can be achieved even with your 32-bit Windows environment by simply increasing your memory from 1GB to just about anything.
So honestly, I'd suggest investing either (a)
$43 for an additional 2GB to install in the second memory slot, or (b)
$78 for one 4GB card to replace your existing 1GB card (remember, 32-bit Windows can only support 4GB of memory). Or, you could even go for 2 of the 2GB cards (for the two slots), as running 2x2GB in "dual memory mode" is somewhat faster than running 1x4GB in "single memory mode".
But really, for just a little investment in added memory you're going to get the best possible bang for the buck in performance improvement. I'm not suggesting upgrading to a a newer/faster hard drive or SSD which is much more costly. Nor am I suggesting you get another machine. I'm just suggesting investing $43 or $78 or $84 in added memory, anticipating you'll genuinely get your money's worth.
I really think this will help.
NOTE: your 320GB is also at the low-end of specs I've seen for the N5050. It's possible your drive might be quite full, and/or in need of a cleanup (deleting all TEMP stuff to free up space, and also to run a DEFRAG possibly). If the drive is just a low-end performing drive, it's hurting you even worse than your 1GB of memory is, because Windows is using the very slow drive to "page" virtual memory in and out of the disk that it doesn't have true "real" physical memory for.