Schoolin' time...
"Slayers" isn't a BIOS. This is an auto installer that makes life easier for many people that are modding an xbox for the first time or generally just aren't good with computers. (Not saying these people aren't smart, just that this isn't their best area.) IIRC this will put the EvoX dashboard on the Xbox and probably an older Xectuer BIOS. Never used it so just going by what I recall reading.
"Avalaunch" is a dashboard. A dashboard is the primary application you will run to access everything on your Xbox... could almost be considered as an OS. Other dashboards are the stock MS one that you get if you boot with no disc (and isn't very useful), the previously mentioned EvolutionX, UnleashX (my fav) and a handful of others. The media center XBMC can also be used as a dash as well, and is probably the sharpest looking one out there.
There is an EvoX BIOS available, but I'll give you two reasons not to use it. First, the Team Xecuter BIOSes are much better products with more features. Second, you're posting in a Team Xecuter forum so those of us that hang out here tend to be partial to it anyway.

The current X2 version is X2CL 5035, it can be found in Xbins. If you haven't used Xbins before check the link in my sig, it tells you how to do so. When you get the BIOS archive there will be two BIOS bin files in there. vOld and v1.6+ or something similar. These refer to what Xbox versions they work with. Since you have a 1.4/1.5 you'll want to use the vOld. Take the time to read the included documentation and the x2config.ini file. The ini is what controls almost all the settings of the BIOS and it should go on the root of your Xbox E or F drive. There is an X2CL forum dedicated for discussing this BIOS, the features and support.
When you get your programmer I would recommend putting X2CL 5035 on bank 1 and another copy of FlashBIOS on bank 2. This is useful if you ever need to reflash just bank 1, you can do so from FlashBIOS on bank 2. Not that you will have much worry with the programmer available...
-Whoopin'