To install MEPIS using the 'Custom install to existing partitions' option that is in the MEPIS installer you first need to create your custom partitions.
You can create the custom partitions using gparted. Gparted is used like a live linux cd. It is downloaded as an iso and burnt as an image.
Use gparted to create the three partitions that you need for an average MEPIS install.
Example: If you have an xp partition, and you want to boot windows and MEPIS, you will use gparted to shrink the xp partition to as small as you need. I usually would leave about 4 or 5 Gb of free space for windows to use. The rest is for MEPIS and maybe some other operating systems too.
If you have a 250 Gb drive then you would keep about twenty Gb in total for windows. Don't make the windows partition too big as you probably won't be using windows much any more and there's no sense in wasting space.
Next you will need a root partition. This partition should be about 7 or 8 Gb but you can do it with 5, 7 or 8 is better. Format the root partition as ext3 with gparted. The root partition is where the configuration files go. After creating the root partition you will need a linux/swap partition. The linux/swap partition should be about 1Gb in size and will be formatted as linux/swap.
This is important. Your fourth partition HAS to be an 'Extended' partition that uses ALL of the remaining space on your drive.
The 'extended' partition can then be divided up into as many logical partitions as you will need in the future. This act gets around the four primary partition limit on hard drives.
From the 'extended' partition you will need to create a logical partition for your home files. Make it as big as you need, at least 50 or 60 Gb in size. Format the home partition as ext3.
You do all of these tasks at once while you are using gparted.
Now you should have a lot of free space in your extended partition to play with. You will use this space later to install as many more linux operating systems as you may want to try out while never messing with your main installs of MEPIS or of MEPIS and windows. More on that later in another thread.
When you install just remember which partition is which. Choose the partitions that you want root. swap and home to be on. It's easy with the MEPIS installer as it will list the partitions by number and by size.
When you install MEPIS you will be asked if you want to install grub to the Mbr of hda or if you want grub in the root partition.
Just this once you can install grub to the Mbr of hda. If you have windows on hda1 then MEPIS will set up so you can boot MEPIS or windows at first boot.
Remember that this is the only time you will install grub to the Mbr of hda. All other operating systems that I will show you how to install later must have grub installed to root. Only the very first install of MEPIS gets to re-write the master boot record. Every 'extra' install, of MEPIS or some other linux, will have grub installed to root. That will keep the Master Boot Record safe.
So for the first install of MEPIS we will install grub to the Mbr of hda.

That's it. Now you can install MEPIS and enjoy the freedom from windows and windows infections. No more virus or spyware scans or virus infections. Linux is safe from those threats.

Or you can now dual boot windows and MEPIS. Nice to have a choice.
I usually say to dual boot when you first try MEPIS. That way you can still play the games and programs that you are used to. After a while you will be bored with windows and will never need it again. Until that time I say dual boot.