There was at one time even a PC Building for Dummy's, or words to that effect lol... Building a PC nowadays is actually a lot simpler then it was with the old 286, 386, and 486 systems. And a darn sight cheaper too lol.
Now, about the single most difficult task is deciding whether to go with AMD or Intel and get the appropriate MoBo. Then decide between DDR or DDR2 RAM and whether to go with SATA for drives or just stick with EIDE. Because most any MoBo (new) out there supports both, and if you chose one that doesn't, getting the expansion card to add the feature isn't THAT hard to do.
Then, you chose your MoBo, but ANY brand of processor would likely fit it. Decide whether to go with SX or DX (Math Co-Processor present or not), then you has the fun of tracking down the type of RAM your chosen MoBo would work with. Parity, Non-Parity, Fast Page, EDO, or any number of various types made to Emulate one style or another. Plus every RAM stick had to match exactly, same type, same style, same speed, and often you couldn't even mix different BRANDS of the same type. Then you had some minor fun with getting the assorted extras like a Floppy Drive and/or CDRom. IBM 3.25 Floppy Drives were mostly considered best, but would not work on anything other then an IBM system. Maybe you still had need for one of the monster 5.25 Floppy drives, even though the discs for them only held about half what a 3.25 holds.
But all told, technicalities aside, even then BUILDING a PC wasn't that difficult. If it doesn't fit there, it doesn't GO there lol... If your chosen MoBo had only 8 bit expansion slots, you wouldn't have much use for 16 bit cards. Although if the MoBo offered VESA slots (used shortly, just prior to the release of PCI) then you could use the VESA cards, or any of the 8 or 16 bit cards you wanted. Then you chose your O/S, while most went with MSDos, there were quite a few offerings out there and most were compatible across most platforms. Chose any extra programs, such as to have Windows or not lol...