If you are refering to the MS PPTP client you need to allow it through PIX/ASA outbound.
go over this link
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a0080094a5a.shtml
Complete these steps to add commands for versions 7.x and 8.0 using the inspect command:
Add PPTP inspection to the default policy-map using the default class-map.
pixfirewall(config)#policy-map global_policy
pixfirewall(config-pmap)#class inspection_default
pixfirewall(config-pmap-c)#inspect pptp
Rgds
Jorge
Jorge Rodriguez