conversi ms-sql7 vs postgresql 7.3

Hi..

I have table xx:
id  debet       credit           balance
1  1000        0                   0
2   2000       0                   0
3         0       2500             0
4         0         100             0

command in ms-sql 7 can use calculate field (column) balance from id=1 to
id=4:
"update xx set bal=balance=bal+debet-credit"
result:
id  debet       credit           balance
1  1000        0                  1000
2   2000       0                   3000
3         0       2500             500
4         0         100             400

How command sql can use in psotgresql 7.3?

thank's a lot
betty.