I have two tables

Table 1 has columns
boat price currency normalprice
fred 1000 Euro ?????

Table 2 has columns
currency exchangerate
Euro 1.2
Pound 1.5

What I want is to create a trigger that when I Insert or Update a row in Table 1 it looks up the right currency in Table 2 and multiplies the price in Table 1 by the exchangerate in Table 2 and stores it back in the normalprice column in Table 1.
I've been having problems in that it updates all the rows in the database not just the one I'm inserting or updating.
Any help would be much appreciated!