1) http://gaim.sourceforge.net among others.

2) Yes. If your Windows partition is FAT32 you can read and write to it from Linux. If it's NTFS, you can only read from it, but that's enough for playing MP3s and such. Lindows may very well have already found and mounted your partition, if it has a good installer. Type

mount

as root, and it'll list all your mounted devices. Look for one that's type vfat or ntfs. If so, you're set already.

To do it yourself from scratch, you need to compile support for VFAT and/or NTFS filesystems into your kernel. It might already be in your kernel, if Lindows comes with a stock kernel. Compiling kernels is a long process, so if you need help with that ask (or find a tutorial).

After you have support for it in your kernel, create a folder where you want to mount your Windows partition. For example, mkdir /mnt/windows (do it as root). Then try

mount -t vfat /dev/hda1 /mnt/windows

Where vfat is the partition type (might be ntfs), /dev/hda1 is the Windows partiton (yours may be different), and /mnt/windows is the mount point.

See also /etc/fstab.

3) All Linux is free, for the most part.