after fresh reinstall of (1) Win XP, and (2) Ubuntu 9.04, differences arose between fat32 bootsector and its backup (dosfsck)

After having some trouble with partitions being ovelapped when I upgraded a dual boot system from 8.10 to 9.04 ( https://answers.launchpad.net/ubuntu/+source/grub/+question/67911 ), I made a backup and started from scratch: formating the whole disk, installed winxp, and partitioned with Ubuntu 9.04beta live CD, and installed it after that.
Just after the installation, everything worked fine.

After some weeks time, I started having troubles again with partitions, as reported at boot time, with a message ssaying "Not automatically fixing this.". :-/

calling manually dosfck I get this (below):

-------------------------------
~$ sudo dosfsck -a /dev/sda1
dosfsck 3.0.1, 23 Nov 2008, FAT32, LFN
There are differences between boot sector and its backup.
Differences: (offset:original/backup)
  430:46/51, 431:61/75, 432:6c/69, 435:20/72, 436:4e/20, 437:54/64, 438:4c/69
  , 439:44/73, 440:52/63, 441:ff/6f, 442:0d/73, 443:0a/20, 444:45/6f
  , 445:72/20, 446:72/6d, 447:6f/65, 448:72/64, 449:20/69, 450:64/6f
  , 451:65/73, 452:20/2e, 453:64/ff, 454:69/0d, 455:73/0a, 456:63/45
  , 457:6f/72, 458:ff/72, 459:0d/6f, 460:0a/72, 461:50/20, 462:72/64
  , 464:73/20, 465:2e/64, 466:20/69, 467:75/73, 468:6e/63, 469:61/6f
  , 470:20/ff, 471:74/0d, 472:65/0a, 473:63/50, 474:6c/72, 475:61/65
  , 476:0d/73, 477:0a/2e, 478:00/20, 479:72/75, 480:65/6e, 481:73/61
  , 482:74/20, 483:61/74, 484:72/65, 485:74/63, 486:0d/6c, 487:0a/61
  , 488:00/0d, 489:00/0a, 506:ba/c6, 507:cb/d7
  Not automatically fixing this.
/dev/sda1: 37648 files, 909218/1918827 clusters
-------------------------------

How can I fix this?

And how could this have happened? I don't want to start having problems again with overlapping partitions...
It looks to me that there is some problem (bug?) with ubuntu grub, or dosfck, ...

More information, in case it helps:
/etc/fstab:
-----------------------------
# /etc/fstab: static file system information.
#
# Use 'vol_id --uuid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
# / was on /dev/sda5 during installation
UUID=0d286bd0-630f-4418-b6b1-cdccc1b9e4c3 / ext3 relatime,error
s=remount-ro 0 1
# /home was on /dev/sda6 during installation
UUID=95a3cabc-a70b-4ab4-986e-08bcfe961c88 /home ext3 relatime
  0 2
# /mnt/sda1 was on /dev/sda1 during installation
UUID=3439-AAAE /mnt/sda1 vfat utf8,umask=007,gid=46 0 1
# /mnt/sda10 was on /dev/sda9 during installation
UUID=418c02cb-5ed1-4e1d-9d70-29984242e1f3 /mnt/sda10 ext3 relatime
  0 2
# /mnt/sda3 was on /dev/sda3 during installation
UUID=5A4E-4D68 /mnt/sda3 vfat utf8,umask=007,gid=46 0 1
# /mnt/sda9 was on /dev/sda8 during installation
UUID=3CA6-89E3 /mnt/sda9 vfat utf8,umask=007,gid=46 0 1
# swap was on /dev/sda7 during installation
UUID=dea6fe36-baf0-41ef-8788-5fd614c49122 none swap sw
  0 0
/dev/scd0 /media/cdrom0 udf,iso9660 user,noauto,exec,utf8 0 0
/dev/fd0 /media/floppy0 auto rw,user,noauto,exec,utf8 0 0
---------------------------------

And the result of sudo fdisk -l:
-------------------
Disc /dev/sda: 120.0 GB, 120034123776 octets
255 heads, 63 sectors/track, 14593 cylinders
Units = cilindres of 16065 * 512 = 8225280 bytes
Disk identifier: 0x0001424b

Dispositiu Arrenc. Inici Final Blocs Id Sistema
/dev/sda1 * 1 3824 30716248+ c W95 FAT32 (LBA)
/dev/sda2 7649 12755 41021977+ f W95 Estesa (LBA)
/dev/sda3 3825 7648 30716280 b W95 FAT32
/dev/sda5 7649 8864 9767488+ 83 Linux
/dev/sda6 8865 10080 9767488+ 83 Linux
/dev/sda7 10081 10323 1951866 82 Intercanvi Linux / Solaris
/dev/sda8 10324 11539 9767488+ b W95 FAT32
/dev/sda9 11540 12755 9767488+ 83 Linux
-----------------------

Any hints?