Speed of computer does not only depends on processor, but also on memory, harddisk, etc.
Celleron is bad processor IMHO. It's simillar to Pentium 2, (400MHz) but with less memory on the processor it self.
You should not play with BIOS if you are not sure what you are doing. Just as someone already suggested: Restore Default Settings.
Download Everest on that computer and check it with it, and post results here.