Here's working code. I checked for invalid input such as the following:

XIr - Invalid Character
XXIVI - Invalid Sequence
XXIVV - Invalid Sequence
XXXXI - Invalid Repitition

Here's the code:
------------------------------------------------------
#include<stdio.h>

int main(void)
{
int i, previous = 0, num, total = 0, flag = 0, key = 0, error = 0, number, repeat = 1;
char roman[50];

printf("Enter the Roman numeral: ");
scanf("%s",roman);

for (i = 0; i < 50; i++)
{

if (roman[i] == 'M')
num = 1000;
else if (roman[i] == 'D')
num = 500;
else if (roman[i] == 'C')
num = 100;
else if (roman[i] == 'L')
num = 50;
else if (roman[i] == 'X')
num = 10;
else if (roman[i] == 'V')
num = 5;
else if (roman[i] == 'I')
num = 1;
else if (roman[i] == '\0')
flag = 1;
else
error = 1;

if (num == previous)
repeat = repeat + 1;
else
repeat = 1;

if (repeat == 4 && num == 1)
{
error = 1;
flag = 1;
}
if (repeat == 2 && num == 5)
{
error = 1;
flag = 1;
}
if (repeat == 4 && num == 10)
{
error = 1;
flag = 1;
}
if (repeat == 2 && num == 50)
{
error = 1;
flag = 1;
}
if (repeat == 4 && num == 100)
{
error = 1;
flag = 1;
}
if (repeat == 2 && num == 500)
{
error = 1;
flag = 1;
}

if (flag == 0)
{
if (num == 0)
error = 1;

if (num > previous)
{
if (key == 1)
{
number = i;
i = 50;
}
total = (total - (2 * previous)) + num;
key = 1;
}
else
total += num;

previous = num;
num = 0;
}
else
{
number = i - 1;
i = 50;
}

}

if (roman[number + 1] != '\0')
error = 1;

if (error == 1)
printf("Error in input.");
else
printf("%d\n",total);

return(0);
}

What is it they say about silence being golden?