constant crashing after mission 14

[AssasS|N_69]

for a while now (ok, its years) ive never been able to finish this game at my house, its pissing me off since i always get this whacked error every time i do something in a mission, moving, killing, shooting anything it will crash on me, i have the update 1.27 but i still get these problems.
just last night i was close to finishing mission 15 but then it crashed when i was saving it, and its really killing the game for me.

can anyone point out the problem?

oh, also

my computer is (in case its the reason why this games screwing up)

p3 500
128mb video card (SiS 128quad with latest drivers)
250 mb of ram (SD ram)
shitty hard drive