Check which version of MDAC you have first:
http://www.microsoft.com/downloads/details.aspx?FamilyID=8F0A8DF6-4A21-4B43-BF53-14332EF092C9&displaylang=en

You can then download the appropriate later version and update.

MDAC 2.8 SP1 (2.81.1117.6) - 2.8 Service Pack 1. The same version of MDAC that comes with Windows XP SP2 
and SQL Server 2000 SP4. 
The MDAC 2.8 SP1 stand-alone release is not supported on the Microsoft Windows XP operating system or on 
the Microsoft Windows Server 2003 operating system. 
To update the MDAC version that is installed together with Windows XP or with Windows Server 2003, you must 
use operating system service packs or Windows Update. 
NOTE: If you find that you need to reinstall MDAC 2.8 SP1 on Windows XP SP2, PQ Systems has found that the 
following non-Microsoft-supported method seems to work.

How to repair a copy of MDAC 2.8 SP1 on Windows XP with SP2 installed. ------------
Right-click on C:\Windows\Inf\mdac.inf and choose "Install". When prompted for the XP SP2 media, point to the 
i386 folder in one of these places: 1. 
The \i386 folder on the XP SP2 CDROM. If you are unlucky enough to have a CDROM that has XPSP2.EXE in the 
root of the CDROM, and no \i386 folder, then you will have to create an i386 folder using a command like this, 
XPSP2.EXE /X:C:\XpSp2 Then point to C:\XpSp2\i386. 2. 
The \i386 folder on a slipstreamed XP SP2 installation CDROM. When prompted for the XP installation media, point 
to the i386 folder in one of these places: 
1. C:\Windows\ServicepackFiles\i386 (it may not like this location, if not, go to the next one) 
2. The \i386 folder on your XP installation CDROM. This reinstalls and repairs MDAC 2.8.