How to write a runtime checksum algorithm for win32 app?

I'm developing an application for windows( WindowsXP, Vista ), and would like to create a function to do an integrity checksum. As the application runs, the checksum function will take a start/end address in the instruction memory and ensure that none of the instructions have changed. The trouble I'm having is that the assembly instructions that take address's, such as JMP, have a different address each time the application runs, since windows changes the base address at which the application resides in memory. How can I create a checksum algorithm that works in this environment? Basically, I'm trying to write my own software protection scheme.Start Free Trial

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:

static unsigned int addressStart;
static unsigned int addressEnd;
_asm mov eax,GUARD_1_BEGIN
_asm mov ebx,GUARD_1_END
_asm mov addressStart,eax
_asm mov addressEnd,ebx
unsigned int a = addressStart;
unsigned int sum = 0;
while (a <= addressEnd) {
        unsigned rawVal = *(unsigned int*)(a);
        sum += rawVal;
        a += 4;
}
if ( sum != 0x00127d69) {
        return 0;
} else {
   // it worked!!!
}