I had Linux and win98 installed on my PC, then I installed winXP and it has replaced the Grub Bootloader, now on booting up I get to see only win98 and winXP to choose from, Now how do I get Back the Grub screen with Linux as a option or how do I boot into linux.

Boot on your GNU/Linux distro's recovery CD or floppy recovery disk you made when you installed your system. Once booted up, mount your filesystem. Once you have your root partition mounted, just chroot to the mounted filesystems. The re run your grub installation.

Bascially, boot up, create a mount point, mount the necessary partitions accordingly. I am not sure of the command to re-run grub, as I only use LILO.

For example after booting the recovery medium:

cd /
mkdir /mnt2
mkdir /mnt/boot
mkdir /mnt/usr

mount /dev/hda1 /mnt2
mount /dev/hda2 /mnt/boot
mount /dev/hda3 /mnt2/usr

chroot /mnt2

Then re-run your grub bootloader as per instructions.

The command to install a GRUB bootloader is

grub-install /dev/whatever

I did it this way
Invoked Grub,

Set the GRUB's root device1 to the partition containing the boot directory, like this:
# grub> root (hd0,0)

If you are not sure which partition actually holds this directory, use the command find,This will search for the file name /boot/grub/stage1 and show the devices which contain the file.
# grub> find /boot/grub/stage1

#grub> setup (hd0) --This command will install the GRUB boot loader on the Master Boot Record (MBR) of the first drive.

If you want to put GRUB into the boot sector of a partition instead of putting it in the MBR, specify the partition into which you want to install GRUB like this:
#grub> setup (hd0,0)

Cheers
Pankaj