the choice is nearly endless - but it can be narrowed down a bit,
ubuntu -
http://www.ubuntu.com/probably the most used desktop linux
mandriva -
http://www2.mandriva.com/linux/one/the linux version i use
pclinuxos -
http://www.pclinuxos.com/my daughter uses this - she's six
all the above are called live cd's, you can use them without installing, they can run in your memory only,
so its 'try before install'
there is also this
http://www.zegeniestudios.net/ldc/ it chooses a linux distribution to fit your knowledge,
security ? - no antispyware or antivirus needed as there a very few to nil nasties for linux,
plus the root folder (roughly the windows folder in xp) is protected by a password that only you know, without the password nothing can write to your root,
multi-chat - there are loads, amsn, kopete, pidgin, kmerlin, etc
torrent - due to the nature of torrent programs, this cannot be discussed, but there are 'built in' torrent programs,
my pc runs mandriva linux - its a multi-media system, i can do anything i want with media files (audio, video, photo, etc)
record, play, transcode, burn to disk, rip home disks, record webcam, photo manipulation - anything
there is no klite codec pack for linux, this is a windows program,
but there are the win32 codecs, which may (depending on the distribution) have to be enabled in software management,
all software you will need is in the repositories of the chosen distribution, all free - its just a matter of knowing what you need and what its called in linux,
eg.
AC-3 (dolby) codec in windows = A52 codec in linux
winamp media player in windows = xmms media player in linux
etc,
anything else then just ask