How to fix a problem in my laptop

hello
my lap top was fully infected with viruses so i decided to reformate it.my computed had only one hard disc only C so i decided to portion it in two picec C and E but when reformating finished i opened my computer and saw that portion E was not display so i started reformating again this thime i reformated only D .the problem that i vave now is that i have tow windows XP home adition instaled in my computer one in C and another one in D and when i switch the computer on it suggest me which of them to chose.
i m concerd how to solve this problem. is it a a danger problem .
please send me a solution as soon as posibile