my partition structure is such......this is a parted o/p
Disk geometry for /dev/hda: 0.000-38204.578 megabytes
Disk label type: msdos
Minor Start End Type Filesystem Flags
1 0.031 5122.287 primary fat32 boot
2 5122.288 38193.596 extended lba
5 5122.318 15359.018 logical fat32
6 15359.049 25595.749 logical fat32
10 25595.780 25697.724 logical ext3
11 25697.755 25956.584 logical linux-swap
12 25956.615 34812.729 logical ext3
7 34812.760 35071.589 logical ext3
8 35071.620 35330.449 logical linux-swap
9 35330.480 38193.596 logical ext3

hda7,8,9 have rh9. hda 10,11,12 have rhel4 WS. i learnt after installing the second linux os that i can have share a single swap between two linux distros. now i want to install linux from scratch in rh9 partitions. i plan to resize hda7 to 100MB and the rest to root partition. and i plan to do it using parted.
but i am in 2 minds...last time i formatted my 10GB linux partition to fat using fdisk i had to format my whole harddisk because i had not used dd command.
now is it safe to use parted without affecting the rest of the harddisk?

"fdisk" always works fine for me.
I'd suggest using GParted instead of Parted, since it's a bit wisier. Although GParted requires Parted libs...
BTW, Parted can't resize Ext3 (only in rare cases).

But actually you can do with only "fdisk" and "mkfs.ext3" (if you want to create Ext3).