Help!

I am using Windows XP. Fairly new pc.

I switched off last night - fine. Switched back on this morning - I've lost everything. The pc seems to have gone back to the way it was when I bought it. The desktop has changed, programs I have installed are no longer there, all of my favourites have gone in IE, my e-mail accounts have gone in OE...........all my e-mail addresses have gone.

All of my website info has gone. I had loads of important info and it's all gone.

Can anyone help me - I'm desperate. Is there any way I can get this info back? What on earth has happened? All I did was switch on in the normal way, then everything just disappeared.

Is there anything I can do? Any help will be so appreciated.