Are you playing the original release of the game (1.49), or the anthology version (1.50x)? (ie a version before or after higher resolution and widescreen support was added)
There was a possible item corruption bug in the original release under Vista/7, which could be avoided by running the game in Win9x compatibility mode (
Inventory graphic placement bug).
If that isn't what you are describing:
For the items that don't equip, have you checked the requirements? Also check the bonuses on the what you are replacing; if the only reason you meet the requirements for a weapon, for example, is a bonus on a weapon that you are currently using, you will not be able to swap for the new weapon.
There are speed penalties and sometimes bonuses on weapons and armour, and bonuses on some jewellery, which are taken into account in the stat display. At the start of the game, for example, the hero has 14 stat points total, but the death knight will appear to only have 13, since speed is reduced slightly by his sword, and the speed stat rounded down to the nearest integer for the purposes of display.
The Reload Time subskill for Bow/Crossbow Expertise also gives a direct boost to speed.