According to http://www.mplayerhq.hu/design7/news.html
, multithreaded support was added to main mplayer last year sometime, but you need to enable it with a command line option, something like:
mplayer -lavdopts threads=N file.mkv
However, since you have a single core P4, I don't think you will get any performance boost. If you run top (or look in system monitor) while playing a 1080p video that you're having problems with, you'll probably see that you're at 100% CPU utilization. Multithreaded in this case would only help if one core was at 100% and another core was sitting there idle.
Another option, since you have a P4 I assume you don't have PCI-express slots. There are some old nVidia PCI cards that support VDPAU (I think it's the 8400 GS), which could probably be obtained on ebay pretty cheap.