[R] covariates in lm

[mrufino@cmima.csic.es]

Dear R list,

I have been trying to do a linear model, extracting the effect of a 
covariate.... and the results do not match, when I do it with other programs 
(e.g. minitab).... so it is obvious that I was doing something wrong.

Whan  I do it with minitab, I have this results: (sector is a factor and depth 
is the covariate):

Source         DF     Seq SS     Adj SS     Adj MS       F      P
sector          6     9.0605     2.9989     0.4998    1.21  0.297
depth           1    34.2072    11.9973    11.9973   29.16  0.000
sector*depth    6     1.5364     1.5364     0.2561    0.62  0.712
Error         578   237.7830   237.7830     0.4114
Total         591   282.5871  


If I do with R, I have been trying everything it occurrs to me and looked 
everywhere and I could not obtain the same results and nothing is clear to 
me... (I am so sorry... probably it is lack of statistical knowledge):

If I do:
> anova(lm(Expr1~depth*sector))
Analysis of Variance Table

Response: Expr1
              Df Sum Sq Mean Sq F value Pr(>F)    
depth          1   38.2    38.2   92.76 <2e-16 ***
sector         6    5.1     0.9    2.07  0.055 .  
depth:sector   6    1.5     0.3    0.62  0.712    
Residuals    578  237.8     0.4                   

I am simply fitting a crossed anova, or because depth is continuos ... what is 
it doing?

then, as it was not right, I went to look in the manuals, and in 'an 
introduction to R' states:
y ~ A + x Single classification analysis of covariance model of y, with classes 
determined by A, and with covariate x. Is it like this?
> anova(lm(Expr1~sector+depth)) #I don't think so...

But I interpreted this as a additive model... and besides it did not work as 
well, so I tried what a friend recomended, i.e. x:z, whereas we are extacting 
the effect of x (covariate) on y... but it does not work as well...
> anova(lm(Expr1~sector+depth+depth:sector)) # Would it be like this?
Analysis of Variance Table

Response: Expr1
              Df Sum Sq Mean Sq F value Pr(>F)    
sector         6    9.1     1.5    3.67 0.0014 ** 
depth          1   34.2    34.2   83.15 <2e-16 ***
sector:depth   6    1.5     0.3    0.62 0.7124    
Residuals    578  237.8     0.4                   
-
or like:  anova(lm(Expr1~depth:depth*sector))


I am lost... in the other times I just did with minitab, but I realy wanted to 
do it with R... can someone give me some lights?
Is it very difficult to do it with R?
Sorry for the long and messy email,

thank you very much in advance,
Marta