Small program, very strange bug

Hi, I have got this incredibly strange bug happening with some code I've written and I fear there could be something fundamentaly wrong with (either me or) my system.

Here's the code (it's short, I promise):

======================================

#include <stdio.h>
#define FILE_NAME &quot;sdtest.dat&quot;

int main()
{
FILE *fp;
long idx = 0;

fp = fopen(FILE_NAME, &quot;r&quot;

if (fp)
{
fread(&idx, sizeof(long), 1, fp);
printf(&quot;old: %d\n&quot;,idx);
idx++;
fclose(fp);
}

fp = fopen(FILE_NAME, &quot;w&quot;

{
printf(&quot;new: %d\n&quot;,idx);
fwrite(&idx, sizeof(long), 1, fp);
}

fclose(fp);
return 0;
}

======================================

The first time I run it I get the following output:

old: 0
new: 1

which increments for each subsequent run, eg.

old: 1
new: 2

then

old: 2
new: 3

etc

BUT, once I have run it enough times for it to get to

old: 25
new: 26

the next run produces

old: 0
new: 1 !!!

The program can not read the file contents when it contains 26! It reads in the file contents of x&quot;1A 00 00 00&quot; as x&quot;00 00 00 00&quot; (verified with debugging).

I have tried this same code on a Unix system at work and there were no problems, what on earth could be causing this? Any insight/advice would be very much appreiated, thanks.

Setup
-----
CPU: AMD Athlon 1GHz
OS: Win 98SE
Compiler: VC++ 6.0