wacazac,
As Valtam said, a clean install would be best in your situation. If you want LL to be only OS, then you'll want to use GParted from the live LL disk to wipe out all the current partitions first. Then use it to create partitions for LL that cover whole disk -- eg. "/" (root) = 15-20GB, "Swap" = 1-2 times amount of RAM, "/home" = rest of disk space. (Here's a tutorial on GParted if you aren't familiar with it:
http://www.dedoimedo.com/computers/gparted.html.)
Not familiar with Mepis myself, but
Distrowatch lists it as a KDE-based distro. If you try to re-use the Mepis /home partition, you'll most certainly end up with problems caused by Mepis configuration files saved there on your new LL OS, which uses Xfce desktop -- so don't try that. Best to backup/copy any data files you want to keep in both Mepis and Windows to an external source (HDD or USB), then copy it back into the new LL /home after install complete.
As for bookmarks/contacts in your XP -- what programs are they associated with? We're assuming that you mean your web browser bookmarks and email contacts. If that's the case, which programs are you using now for those?
If you're using Firefox web browser and Thunderbird email in Win XP, then it should be very easy for you to keep everything as it is now. Do web search for how to copy the profile folders of those from Windows to a USB/HDD. Then you can just paste those folders into your /home/username folder on the new LL installation and have everything instantly as it was.
If you're using MS Internet Explorer and Outlook (or similar), then easiest thing to do might be to install Firefox and Thunderbird into Win XP and import everything from IE and Outlook into those programs. Then, after confirming that the import worked properly, copy off the profile folders for Firefox and T-Bird to be used in LL. (I'm not familiar with MS programs so can't offer guidance on exporting/importing, but web search should easily turn up answers for that.)
Feel free to post back with more questions if necessary. Also, let us know your approximate level of experience with Linux so we know don't end up wasting your time describing basic proceedures that you already know well.