When a system of small magnetic particles is not very much diluted, the susceptibility peak arises from a delicate combination of individual blocking behind anisotropy barriers and collective freezing due to dipolar interactions. We have studied a sample of 4 nm Permalloy particles in alumina (25% concentration), and measured the frequency dependence of the ac susceptibility. Our scaling analysis shows that cooperative freezing effects do indeed contribute significantly to the blocking temperature; accounting for the dipolar interactions, the value obtained for individual anisotropy barriers is 10 times lower than that crudely deduced from the blocking temperature. No clear evidence of infinite-range correlations is found, in contrast with the case of spin glasses. We have studied the relaxation of the zero-field-cooled magnetization with similar procedures as used to characterize the slow dynamics of spin glasses. We find that the relaxation is weakly sensitive to waiting time effects, and is more strongly influenced by the cooling time, as is usually quoted for structural glasses.