Login Admin Password Hijacked at computer startup?

I have a admin password set up to start my dell laptop using vista home premium.  Up to now it has worked fine.  This morning I go to logon and get my computer up and running and the password appears to have been changed.  Previous password was 10 letters and the new logon password looks to be 4 letters.  I'm the only person that has access to the computer and we have not had any network problems at our house.  How would I go about solving this without re-installing vista.  I got frustrated and tried to re-install vista but not being able to type the correct password in my  computer just tells me I have a incorrect password.  I'm not able to use any keys like f2 or f12 on restart, and the computer would not start my vista disk either.  Have I been hijacked by someone outside my little home network.  What do I do.

Thanks for any help.

Jim