Before all, can you complete your profile, its more friendly and we can answer with more enphasys!
May be you dont configure your BIOS to boot from CD/USB.
Actually there is a lot of version of BIOS, and In each one, there is different ways to get in to change your config.
Try this:
When you power-on your PC, the first screen/image that you can see is about motherboard (before windows logo), when this appear, you can press some of this keys to get in Setup mode:
F2, F4, F8, Delete key when LOGO or Info system appear to enter in setup.
In some BIOS, you can read some info about keys near to buttom or may be upper in left side.
F12 or F10 in some system let you select BOOTING media. (A list, to Select you booting drive)
If you can GET IN, you must search in menu only "BOOTING SECUENCE" or "BOOTING DRIVE" or similar stuff...
Let be first USB/CD (some can change with +/- key another with page up/down or with cursor keys).
Read at buttom there is info about key and How can you save all change. Take care with configuration changed, I recomends you write every parameter changed.
Often you can save that pressing F10 and accept.
-EDIT-
**I see, so, May be your Grub its not working anymore, so you need download a tool to recover it, and reinstall it.
You can try in Windows, downloading Wingrub, you can follow all instructions from here: http://members.iinet.net.au/~herman546/p9.html
Tell me if you resolve your problem!
Bye.