'Missing Operating system'- Windows Vista

help me out here guys...

recently i bought a dell computer installed with Windows Vista Business, but
it's been giving me problems lately
- lagging a lot (i think its due to the hardware)/(e.g. opening folders, IE,
msn)
- can only run two programs at the same time, if more it will crash
- when i attempt to use another program while downloading something, it
would hang (i d/led iTunes, and opened the IE, it hanged!)

The worst thing that happened was when i was updating the computer, and I
attempted to open a cd. Suddenly the whole computer crashed and when i tried
to switch on my CPU, it shows 'Missing Operating System'

So just a few inquiries
- Could this be due to the hardware itself, or the operating system?
- Is there anyone with the same issue?
- Are there any known bugs with similar incidents?

Thanks

One more inquiry: During startup(when the bar is loading), do you see a
Vista logo?
Cos my computer doesn't when i do so