Is it really worth it?

It depends on what you can do with it. It also depends on what you wish to acomplish. Some effects cannot be obtained with the P4 renderer.

There's no absolute answer.

As for reinstalling the OS, I'm affraid that in some cases that the only solution. Indeed most of the Poser users are not especially versed in their system security or stability. Many people install little things without even knowing what they're doing. Some nasty background softwares, such as tool bars, funny cursors, XP tweaksers can install themselves silently. Some great company packages can even interfere with the stability of Poser, such as QuickTime or wrong drivers.

In the end, it is easy to get a machine that can't run a thing even with a strong hardware configuration, the windows registry being full of craps.

In such case, the best way is to restart from a fresh OS install. You'd be amazed how fast your computer can be then. ;)