Whether doing IPSec or PPTP (you don't specify), the problem is that you're probably doing PAT on the PIX. PAT and IPSec/PPTP don't work well together, particularly through a PIX (in fact, 6.3 code due out next year will have support for one IPSec and one PPTP tunnel going through a PIX with PAT, but that doesn't help you much now).
The only way around this at the moment is to create a static one-to-one translation for your inside PC on the PIX, but of course that means you need a second global IP address. If you're coming from the inside, you'll also need to create conduits/ACL's to allow IP protocol 50 (IPsec, actually ESP) or IP protocl 47 (PPTP, actually GRE) to come back in, cause the PIX won't open a hole for these automatically cause they're not TCP/UDP protocols.
