Hey guys, does Steam.exe ever just close it's self? No crash report, nothing? Well, I've compiled a short list of things you can do, short of re-installing it altogether.
Try doing these things, one by one. Make sure that steam is off (check task manager). After each step, re-launch steam again.
1: Delete ClientRegistry.blob (Found in your steam folder). If you can not find it, use the search function with in the steam folder. C:\Program Files\Steam
2: Delete everything, except steam.exe and steamapps folder.
3: Delete Appcashe.
4: Defrag hard disk.
5: Optimize the space on hard disk.
Keep in mind: this isn't an all at once thing. You need to do them one by one. Do a step, re-launch steam. Wait to see if steam closes it's self, or even crashes. Do the next thing, etc.
This is most of what valve will tell you to do in an error/ticket report. Short of deleting things from your actual registry. NO ONE should mess with their windows registry, unless you know exactly what it is that you're deleting. If you don't know, have a friend do it for you. Deleting the wrong thing from any part of your windows registry can, and will corrupt windows. I've seen people delete system 32 before, and it made me laugh inside. However, it's no laughing matter to have to re-install windows. So, just be cautious.