I have a system with Windows Xp on C: and Windows Vista on F:
Now I want to install SUSE 10.1. What problems I can Expect. What precautions should I take so that I can run all OSs safely.
Earlier I used to have XP and SUSE running finely.

I had no problems.
Just configure LiLo/GRUB properly.

Make sure that all Microsoft products are installed on the drive first as they refuse to recognize Linux partitions. Also the MBR must be on the first partition so that Windows will start properly.

Also if you intend to share files between these OSes, you should format a common partition as FAT32, because NTFS read/write (I/O) operations under Linux is experimental at best.

This is the way I would partition your drive:

PARTITION 1--NTFS Windows XP
PARTITION 2--NTFS Windows Vista (assuming you are using the beta version)
PARTITION 3--FAT32 Shared Data Partition (between Windows and Linux)
PARTITION 4--SWAP Linux Swap Space (usually 2x the RAM istalled)
PARTITION 5--ROOT REISERfs Linux Programs and Root Directory (20GB)
PARTITION 6--HOME REISERfs Linux USERS and Data Partition (remaining space on drive)

It is easiest to create 4 partitions when you install windows. Leave the 4th partition blank or unformated so that Suse YaST can be installed on the free (unformatted/uninitialized) space. Suse will partition its own space accordingly and it will install GRUB to replace the NTLDR of the MBR.

I know this is a little confusing, if you have any questions please ask.