Explain following declaration int *P(void); and int (*p)(char *a);

What is external and internal variables What is dynamic memory allocation what is storage classes in C

I have got one problem .I initially have got windows vista in my computer but now I installed linux UBUNTU operating system into my computer .For this i first format D: drive and got installed finely there and restart , when it boots there there were two options vista and ubuntu .But the problem is that when i choose vista it opens but screen seems to be dark only mouse pointer sees in the screen and no more windows appeared . But when i choose ubuntu it opens and asks for username and password .But i don't know these things. But linux is freeopen software , why it asks userename and password ? Now i am unable to use my computer ,i installed lunux without partioning the drive. Plz give me the solution The computer configuration of my computer is 80 GB harddisk, 1 GB RAM.3GHz speed.