The i in i915p, and i915g, stands for Intel.
It is a motherboard chipset made BY Intel, for IBM/Lenovo.
HOWEVER, just as you suspected, the drivers are proprietary from Lenovo.http://en.wikipedia.org/wiki/List_of_intel_chipsets#Pentium_4.2FPentium_D.2FPentium_EE_chipsets
Research has led me to conclude the Lenovo 29r8260 motherboard, was used in the IBM/Lenovo ThinkCentre M51; series of Desktop PC's.
Let's go see if this holds true, using the information we have.......http://reviews.cnet.com/desktops/lenovo-thinkcentre-m51/4507-3118_7-31000439.html
Intel 915G motherboard chipsethttp://www.ebay.com/itm/IBM-ThinkCentre-M51-Motherboard-3-4GHz-P4-HT-29R8260-8143-WMG-/360578959486?pt=US_Motherboard_CPU_Combos&hash=item53f42e487e
So far everything looks good.
One more thing to check.........
Go to Device Manager, and see if Broadcom is listed; under -
(Using WinXP as an example,
Start > Right-click on My Computer > Left-click on Properties at the bottom of the list.
Left-click on the Hardware tab > Left-click on Device Manager.
Left-click on the + sign next to - Network adapter)
If Broadcom is the name of the ethernet chipset, it will be in the Network adapter list.
If this holds true, then the following Lenovo Support website, for the ThinkCentre M51 is the one you want,http://support.lenovo.com/en_US/downloads/default.page?selector=expand
Using the Ethernet driver as an example;
1) Click on - Networking: LAN (Ethernet)
2) Click on the blue - [+] Click for files
3) Click on the blue - Broadcom Ethernet driver and software.
(Or Download, to the right)
4) A small window will come up -
Click on Save File.
5) A small download window will come up.
Allow the file to download all the way, then DOUBLE-click right on the file name.
At the bottom of the next 3 small windows, click on -
Next, Next, and Finish.
If you are not asked to restart your computer, do so.
Close all windows, go back to your desktop screen.
WinXP: Click on Start > Turn Off Computer > Restart.
I'm sure you didn't need all that, but included just in case.
For additional questions please post in a Comment, or on Reply at the end of the solution.