It could be a lot of things but maybe you could try this?
If it's a software issue...
- Try launching STEAM in Windows Safe-mode (w/ Network) and see what happens.
- Disable (at least for testing) all the non-essential start up programs.
If that fails then you might want to test your hardware...
- Run a system RAM check...any errors?
- Run a Hard Drive check (for paging errors)...any errors?
Let us know how it goes.