If U press F8 at startup(if it doesn't work the first time, keep pressing it), can U get to safe mode , or last good known config ? If so, U can do a system restore.
If not, try removing the back battery, then put it back in. Start it again.
If not, can you get in the bios screen ? (F2 or dell usually)
Did U had any errors recently, made any major changes?
Do U have an xp cd?