I have laptop where I have installed Xandros 3.o OS and windows XP. I would like to remove Xandros and
install Knoppix 5.0 instead.I need to know how I can uninstall it. I'm a linux newbie.Please tell me how to do it in a easier manner.

Thanks in advance

just install over it.

okay did you 1st install XP?
then 1st of all why do you want to use Knoppix and not something like FEDORA Core?
If you're really a Linux starter then I would recommend to use not a live distri like Knoppix. Better use something where much more users have run already in problems that have been solved or is already talked about like Suse or Fedora.
But never the less. Just Boot with your favorite Distri and for example if it's Fedora then you can say at the partitioning part of the setup to clear out all linux partitions. The standard install of FC5 does a good partitioning for starters and reconfigures Grub Bootloader so that you still will have multiboot with FC5 and WinXP.
regards

I installed Windows XP first and then Xandros.But when I installed Xandros it did not create partitions.I think it recognised the windows partions.I can't use Fedora Core because I have a Pentium II PC and is a bit slow.Fedora Core and SUSE require higher speeds.

No you can use it.
Here are the offical ones:

6.2.1. x86 Hardware Requirements

In order to use specific features of Fedora Core 5 during or after installation, you may need to know details of other hardware components such as video and network cards.

6.2.1.1. Processor and Memory Requirements

The following CPU specifications are stated in terms of Intel processors. Other processors, such as those from AMD, Cyrix, and VIA that are compatible with and equivalent to the following Intel processors, may also be used with Fedora Core.

* Minimum: Pentium-class — Fedora Core 5 is optimized for Pentium 4 CPUs, but also supports earlier CPUs such as Pentium, Pentium Pro, Pentium II, Pentium III, and compatible AMD and VIA processors. Fedora takes this approach because Pentium-class optimizations actually result in reduced performance for non-Pentium class processors. In addition, scheduling for Pentium 4 processors, which make up the bulk of today's processors, is sufficiently different to warrant this change.
* Recommended for text-mode: 200 MHz Pentium-class or better
* Recommended for graphical: 400 MHz Pentium II or better
* AMD64 processors (both Athlon64 and Opteron)
* Intel processors with Intel® Extended Memory 64 Technology (Intel® EM64T)
* Minimum RAM for text-mode: 128MiB
* Minimum RAM for graphical: 192MiB
* Recommended for graphical: 256MiB

I've never used Xandros and Knoppix, but all other Linux re-installations work like this and you don't need to "uninstall" them.
Probably inst-1 (installation 1) has created some partitions - could you post /etc/fstab please? During inst-2 (installation 2) these partitions will be found by the installation media. You can either
-delete them and create new partitions on the free space
-keep them and install over them
-shrink these or other partitions (ntfs maybe) in order to create free space, then create linux partitions there and install.

These all depend on how automated the installation tools offered by each distro are... If you want to search further and you don't have a problem destroying your installations for the sake of knowledge you can do all these manually. Commands/tools like like fdisk, cfdisk, ntfsresize, mke2fs/mkreiserfs parted would be useful and are used by the installer programs of many distros.

Run fdisk -l as root and post here

Why should he do all this?
See what acid_kewpie or nocturna_gr said.
Why should we give him a hint on how to tinker on his system if every insaller of a common distri will recognized?

The fdisk will probably help so we can tell him which partition not to install on if he wants to retain his XP.

I will buy Fedora Core 5.But I hope it is not slow because the minimum speed requirements for Graphics is 400Mghz+. And my computer's speed is 350MgHZ with 256MB Ram. Anyway I will also try Fdisk.

Thanks for your advice!