This is a weird one. The only thing I can think of at this point is to eliminate XP's looking for the tag file.
Try editing TXTSETUP like this:
CODE
[Version]
signature="$Windows NT$"
ClassGUID={00000000-0000-0000-0000-000000000000}
[SourceDisksNames]
_x = %cdname%,,,""
_1 = %cdname%,,,""
_2 = %cdname%,,,""
_3 = %cdname%,,,""
_4 = %cdname%,,,""
_5 = %cdname%,,,""
_6 = %cdname%,,,""
1_ = %cdname%,,,""
2_ = %cdname%,,,""
3_ = %cdname%,,,""
4_ = %cdname%,,,""
5_ = %cdname%,,,""
6_ = %cdname%,,,""
[SourceDisksNames.x86]
1 = "Windows XP Professional CD-ROM",,,\i386
2 = "%cd2name%",,,\cmpnents\tabletpc\i386
3 = "%cd2name%",,,\cmpnents\mediactr\i386
4 = "%cd2name%",,,\cmpnents\netfx\i386
_1 = %bootname1%,,,""
_2 = %bootname2%,,,""
_3 = %bootname3%,,,""
_4 = %bootname4%,,,""
_5 = %bootname5%,,,""
_6 = %bootname6%,,,""
1_ = %bootname1%,,,""
2_ = %bootname2%,,,""
3_ = %bootname3%,,,""
4_ = %bootname4%,,,""
5_ = %bootname5%,,,""
6_ = %bootname6%,,,""
7 = %cdname%,,,\i386\lang
8 = %cdname%,,,\i386\lang
9 = %cdname%,,,\i386\lang
10 = %cdname%,,,\i386\lang
11 = %cdname%,,,\i386
12 = %cdname%,,,\i386
13 = %cdname%,,,\i386
14 = %cdname%,,,\i386
15 = %cdname%,,,\i386
107 = %cdname%,,,\i386\lang
1984 = %cdname%,,,"\i386\NLDRV\001"
1985 = %cdname%,,,"\i386\NLDRV\002"
1986 = %cdname%,,,"\i386\NLDRV\003"
IMPORTANT NOTE! Windows 2000 and XP both still look for the CD tag files in many of their driver INF files. So if you get the insert SP2 CD error further along in the setup process, you have to tackle the INF files in the devices to install section of SYSSETUP.
I've seen (and solved) this error dozens of times, but it's strange that you guys keep seeing it. It might be a driver INF after all.