I recently built a PC: on the BIOS screen I am being shown two messages: 'CMOS settings wrong' and 'CMOS memory size wrong'. Also I think because of this I can't boot windows from the CD rom. WHAT do these messages mean?!

Whenever you build a machine & boot for the 1st time, you should go directly into the BIOS (aka CMOS) & configure the settings to match the hardware you have installed. Read your motherboard manual...there should be a detailed section on the BIOS ASUS A7N8X-X Athlon XP 1800+ 8.5 x 200MHz 1024MB PC3200 2.5-3-3-7 Asus A9550GE/TD 128MB WinME/WinXP Pro SP1

Jam's advice is good. If that doesn't help, try resetting the BIOS back to factory default settings by using the jumper on the mainboard. If you can't find the jumper, just pull the BIOS battery for 20" and then snap it back in. Sometimes the settings get messed up when they are testing the board at the factory and resetting back to default get you booting. Then you can go back in and custom set the CMOS if you wish. Give a man a fish and you feed him for a day; Teach a man to fish and you feed him for a lifetime; Then industry pollutes the water and kills all the fish.