It is just about ALWAYS a good idea to uninstall any and all previous codec packs, before installing a new one. If you keep doing "dirty" installs, eventually you are quite likely to experience some rather odd issues with codecs ^^' .
Also, it's probably a good idea to NOT use LAV filters (if offered with w/e codec pack you settle on; I use the K-Lite codec pack) on an older, weaker PC; they're higher quality but can be a significantly heavier CPU load. Try using ffdshow instead, if you haven't already. Ffdshow also has a significantly better UI than the LAV filters, imo.