Adding new Windows 7 computers to SCCM and AD but PXE botting with Windows XP

Currently I have 500 windows XP workstations in SCCM and within AD sorted in to different OU's. SCCM is setup with "Active Directory System Group Discovery" to query workstations OU to update SCCM. In SCCM I have 48 collections. 
 
This week I removed 50 old computers and then replaced with newer computers from different rooms. This was fairly easy as I copied over the "members of" from the old computers to the newer computers then deleted the old computers from AD. Logged on each computer and changed computer name to the old computer name. I then "updated membership collection" in SCCM which picked up there new names and new collections from AD.
 
I now have 112 new HP8300 Elite windows 7 PC which I would like to add to SCCM 2007 and AD but they need to be PXE to windows XP first I think.
 
How would you go about adding these new computers to SCCM and AD
 
ALSO I think this is now a major issue (as my "all user workstations" has not updated for two days)
 
I just went to site status - school name - component status and everything is OK except for SMS_AD_SYSTEM_GROUP_DISCOVERY_AGENT (offline) and If I click show all messages the errored one is 5503
 
SMS Active Directory System Group Discovery Agent reported errors for 23 objects. DDR's were generated for 0 objects that had errors while reading non-critical properties. DDR's were not generated for 23 objects that had errors while reading critical properties.

Possible cause: The SMS Service might not have access to some properties of this object. The container specified might not have the properties available.Solution: Please verify the Active Directory schema for properties that are not replicated or locked. Refer to the discovery logs for more information.
 
What do I need to do as I have no clue on how to solve this problem after endless amounts of searching.
 
Any help would be so much appreciated.
 
My Story: Im completely new here and to networking as two months ago I started at a school as a trainee systems administrator which I knew nothing about networking and now the system administrator has left and a replacement starts next month but until then I am the new only systems administrator for the school with 100+ new computers to install within the next month. Talk about getting put in the deep end.

As part of your task sequence to build new PCs you should have a step to add the computers to AD - google this (if not, you could always add them manually) and they should also be added automatically to SCCM when the client is installed as part of the task sequence
 
If the PCs are already built, just add them to AD, let SCCM discover them through AD system discovery  and then you could squirt out the client agent to them by right clicking on the computername in sccm and install client.

I already have the new PXE image (PXE-HP8300 XP) ready in SCCM.
 
As the new computers are windows 7 I need to first add them to SCCM so they can install XP then add them to AD
 
This is the steps I have been doing so far:
 
1. "import single computer" to SCCM by adding mac address and new computer name
2. I add the computer to the PXE-HP8300 XP collection
3. I then F12 computer so it PXE boots which it then installs XP fine.
4. Once computer has finished installing XP, the computer then shows up in "computers" in "AD"
5. I drag the computer in AD to the correct OU and add correct "members of" to computer
6. SCCM updates collection "all user workstations" with new computer
 
This all seemed to be working fine BUT if I go to the client machine there is no "configuration manager" installed in control panel and also it does not pick up advertisements.
 
If I right click properties then advertisements on the computer in SCCM it shows up about 20 advertisements but not installed any on the client
 

Have a look at the C:\windows\system32\ccmsetup\ccmsetup.log on the clients to look for any errors