The Signal Processing Blockset that are trying to install requires Simulink and is useless without Simulink. It does not affect use of the Signal Processing Toolbox by itself, so you should only install Signal Processing Blockset if you install Simulink.
It should not have been possible to order Signal Processing Blockset without ordering Simulink, but something might have slipped up.
In R2011a (after the R2008b version that you are working with), Signal Processing Blockset and Filter Design Toolbox were merged to create DSP System Toolbox, but DSP System Toolbox only recommends having Simulink rather than requiring it.
This leads to one possibility: that you have a license for DSP System Toolbox because you bought it for R2011a or later, but you do not have a license for Simulink, and for some reason you are going back to an older MATLAB release. The licensing software that determines which old toolboxes you are permitted to download would know about your DSP System Toolbox and know that Signal Processing Blockset is an older version of it, so it might allow Signal Processing Blockset to show up in what you are allowed to download. But Signal Processing Blockset required Simulink and if you never had Simulink for that license (at least not for R2008b or later) then it might not allow you to download Signal Processing Blockset.
If that is what is going on then you do not need Signal Processing Blockset for your purposes, as you would only have been using the non-Simulink facilities of DSP System Toolbox and those facilities were in Filter Design rather than in Signal Processing Blockset.
R2011a also merged Communications Blockset (which was for Simulink only) and Communications Toolbox, producing Communications Systems Toolbox, so you could run into the same problem if you tried to download the Communications Blockset along with the Communications Toolbox.