The problem is, that a microphone requires a pre-amp, which means you can't run it through the line-in on the motherboard through a splitter, without any kind of pre-amplification. The motherboard mic-in has a built-in pre-amp, and the stereo line-in doesn't.
Also, the "line" signal from the guitar, and the line-in on a motherboard is not directly compatible, so you
can't connect a guitar directly to the line-in on a motherboard, it simply wont work without an audio interface between the guitar and the PC.
The best solution for your requirements would be a 2 channel USB soundcard, which has built in pre-amps for a microphone and also accepts a signal from a guitar.
Something like this
dual input soundcard or
this (or similar) would be adequate, and the inputs can be set to microphone or line-in, and it accepts both unbalanced and balanced line signals
For recording software, you could use something like
Audacity, which is free, and allows you to record the guitar and the mic on separate tracks, so you can use different effects on each channel
EDIT:
Apparantly someone has invented t
his (guitar) line to USB adaptor since I last checked. It is a lot cheaper, and does the job of a di box / line driver (on the cheap), but doesn't let you record vocals (mic) at the same time, unlike the USB sound card.