Actually, DIMM and SDRAM are not to be compared. DIMM (dual inline memory module) is a packaging standard. SDRAM (synchronous dynamic random access memory) is a standard according to which the memory module is synchronized with the front side bus. PC133 is an SDRAM DIMM and PC2100 is also an SDRAM DIMM. Difference is PC133 is single data rated (hence SDR SDRAM) and PC2100 is double data rated (hence DDR SDRAM). SDR = data is transferred once per clock cycle. DDR = data is tranferred twice per clock cycle. Your "bro's" system must be using a DDR system too, since a PC2100 module wouldn't fit in a PC133 slot. DDR modules have two notches as opposed to SDR which have only one, unless I'm just fat wrong.
Anyhow, make sure that all the parts are firmly in place and that BIOS settings are OK. If you hear some beeps at bootup and the PC freezes, somethings are definitely wrong in your hardware.
<font color=red>A platform is not an oil rig.</font color=red>