laptop keyboard not working unless On-Screen Keyboard is Opened

I am Using Dell Inspiron 1545N laptop having Windows 7 Ultimate 32 Bit OS, since a couple of days i am having a strange problem with my laptop keyboard , it is not working at startup but once fully logged in i have to open on screen keyboard to make it work. earlier i assumed it as a hardware problem, since it works after opening onscreen keyboard it might be software issue. because of this i am unable to press F8, F12 and F2 at startup, and even i disabled password login as i am unable to enter password at login screen. can anybody please help.

Hi Paul and Megahertz07 , thank you both for responding , Paul as you said its not drivers issue as i uninstalled them and reinstalled with latest version, still the above said problem is there. physical keyboard is working fine, what i have to do after logging in is, i just have to open onscreen keyboard and close it again so then only physical keyboard is working, till then its not responding.
And Megahertz07 i performed Diagnostics using Dell Support Assist and found out that Right Alt key is not working, other than that under my observation no liquid spilled and i take good care of my laptop, after completion of that particular days work, i logged off and went to bed and the next day when i opened it keyboard is not working.
i just want to know will that be the case when a single failed key makes entire keyboard not to work at startup and how the same keyboard is working after logging in once i open and close on screen keyboard?
Thanks in advance and suggest me the next course of action and also let me know if a single key can be replaced or i have to change entire keyboard if there is no other solution..

As I wrote, if it's not working at startup, it's not a windows / software problem.
As you already have Right Alt key that is not working, I would say it's a hardware problem.

Megahertz07 said:

As I wrote, if it's not working at startup, it's not a windows / software problem.
As you already have Right Alt key that is not working, I would say it's a hardware problem.

How is that a Hardware problem? His keyboard physical keyboard STARTS TO WORKS once on screen one is opened. It is clearly a software related issue. If it was hardware problem, then his keyboard wouldn't work at all regardless of opened on screen one.