switching between Basic & Aero interfaces

>I was in the midst of playing around on my computer when it decided it
>needed
> to switch to the Vista Basic. I restarted the computer and it was still
> in
> basic. So I went into display properties and I set it back to the Aero
> interface. But whether I hot OK or Apply, it stays in Basic. It will not
> go
> back to the Aero interface. I miss my pretty colors and 3D mode! How can
> I
> force it back to Aero?
>
> I have Vista Home Premium. I upgraded over XP Media Center 2005
>
> Here's my setup:
> Intel Core 2 Duo Processor, 2.40 GHz
> 256 MB video card
> 320 GB hard drive
> 2 GB RAM
>
> Computer was purchased from Dell in December of 2006.
>
> Thanks in advance.