Replacing the Boot Sector
If Ntldr is damaged or missing, or if the boot sector is corrupted, you can resolve either problem by using the Recovery Console
Using Recovery Console to Recover from Startup Problems
Troubleshooting Disks and File Systems
Windows® XP Professional provides improved troubleshooting tools for disks and file systems
According to where I found this info,the problem can be cured by altering the boot sequence.If this is correct(and you can)then the reason its often happening on your pc is caused by boot order of devices.(CD Floppy HDD etc)The NTLDR missing is not likely to be caused by an infection,its most probably due to you changing the order after reinstall.