Using the latest of FF and Sync Preview on both PC's. The primary PC has Vista Ultimate and the second PC has XP SP3.

Second PC the sync fails. Debug output from the end of the file is:
zotero(5): Transaction in progress -- increasing level to 2

zotero(5): SELECT COUNT(*) FROM tags WHERE tagID=?

zotero(5): Binding parameter 1 of type int: 3246

zotero(3): Saving tag 3246

zotero(5): INSERT INTO tags (tagID, name, type, dateModified, key) VALUES (?, ?,
?, ?, ?)

zotero(5): Binding parameter 1 of type int: 3246

zotero(5): Binding parameter 2 of type string: "Pornography"

zotero(5): Binding parameter 3 of type int: 1

zotero(5): Binding parameter 4 of type string: "2008-09-04 20:24:45"

zotero(5): Binding parameter 5 of type string: "38VTN4X5"

zotero(5): Flagging nested transaction for rollback

zotero(5): Rolling back transaction

zotero(3): Resetting Notifier event queue

zotero(3): Loading all collections

zotero(5): SELECT C.*, (SELECT COUNT(*) FROM collections WHERE parentCollectionI
D=C.collectionID)!=0 AS hasChildCollections, (SELECT COUNT(*) FROM collectionIte
ms WHERE collectionID=C.collectionID)!=0 AS hasChildItems FROM collections C

zotero(3): Unloading all creators

zotero(3): Loading all items

zotero(5): SELECT I.*, COALESCE(CASE (SELECT COUNT(*) FROM itemCreators IC LEFT
JOIN itemTypeCreatorTypes ITCT ON (IC.creatorTypeID=ITCT.creatorTypeID AND ITCT.
itemTypeID=I.itemTypeID) WHERE itemID=I.itemID AND primaryField=1) WHEN 0 THEN N
ULL WHEN 1 THEN (SELECT lastName FROM itemCreators IC NATURAL JOIN creators NATU
RAL JOIN creatorData LEFT JOIN itemTypeCreatorTypes ITCT ON (IC.creatorTypeID=IT
CT.creatorTypeID AND ITCT.itemTypeID=I.itemTypeID) WHERE itemID=I.itemID AND pri
maryField=1) WHEN 2 THEN (SELECT (SELECT lastName FROM itemCreators IC NATURAL J
OIN creators NATURAL JOIN creatorData LEFT JOIN itemTypeCreatorTypes ITCT ON (IC
.creatorTypeID=ITCT.creatorTypeID AND ITCT.itemTypeID=I.itemTypeID) WHERE itemID
=I.itemID AND primaryField=1 ORDER BY orderIndex LIMIT 1) || ' and ' || (SELECT
lastName FROM itemCreators IC NATURAL JOIN creators NATURAL JOIN creatorData LEF
T JOIN itemTypeCreatorTypes ITCT ON (IC.creatorTypeID=ITCT.creatorTypeID AND ITC
T.itemTypeID=I.itemTypeID) WHERE itemID=I.itemID AND primaryField=1 ORDER BY ord
erIndex LIMIT 1,1)) ELSE (SELECT (SELECT lastName FROM itemCreators IC NATURAL J
OIN creators NATURAL JOIN creatorData LEFT JOIN itemTypeCreatorTypes ITCT ON (IC
.creatorTypeID=ITCT.creatorTypeID AND ITCT.itemTypeID=I.itemTypeID) WHERE itemID
=I.itemID AND primaryField=1 ORDER BY orderIndex LIMIT 1) || ' et al.' ) END, CA
SE (SELECT COUNT(*) FROM itemCreators NATURAL JOIN creatorTypes WHERE itemID=I.i
temID AND creatorTypeID IN (3)) WHEN 0 THEN NULL WHEN 1 THEN (SELECT lastName FR
OM itemCreators NATURAL JOIN creators NATURAL JOIN creatorData WHERE itemID=I.it
emID AND creatorTypeID IN (3)) WHEN 2 THEN (SELECT (SELECT lastName FROM itemCre
ators NATURAL JOIN creators NATURAL JOIN creatorData WHERE itemID=I.itemID AND c
reatorTypeID IN (3) ORDER BY orderIndex LIMIT 1) || ' and ' || (SELECT lastName
FROM itemCreators NATURAL JOIN creators NATURAL JOIN creatorData WHERE itemID=I.
itemID AND creatorTypeID IN (3) ORDER BY orderIndex LIMIT 1,1) ) ELSE (SELECT (S
ELECT lastName FROM itemCreators NATURAL JOIN creators NATURAL JOIN creatorData
WHERE itemID=I.itemID AND creatorTypeID IN (3) ORDER BY orderIndex LIMIT 1) || '
et al.' ) END, CASE (SELECT COUNT(*) FROM itemCreators NATURAL JOIN creatorType
s WHERE itemID=I.itemID AND creatorTypeID IN (2)) WHEN 0 THEN NULL WHEN 1 THEN (
SELECT lastName FROM itemCreators NATURAL JOIN creators NATURAL JOIN creatorData
WHERE itemID=I.itemID AND creatorTypeID IN (2)) WHEN 2 THEN (SELECT (SELECT las
tName FROM itemCreators NATURAL JOIN creators NATURAL JOIN creatorData WHERE ite
mID=I.itemID AND creatorTypeID IN (2) ORDER BY orderIndex LIMIT 1) || ' and ' ||
(SELECT lastName FROM itemCreators NATURAL JOIN creators NATURAL JOIN creatorDa
ta WHERE itemID=I.itemID AND creatorTypeID IN (2) ORDER BY orderIndex LIMIT 1,1)
) ELSE (SELECT (SELECT lastName FROM itemCreators NATURAL JOIN creators NATURAL
JOIN creatorData WHERE itemID=I.itemID AND creatorTypeID IN (2) ORDER BY orderI
ndex LIMIT 1) || ' et al.' ) END) AS firstCreator, (SELECT COUNT(*) FROM itemNot
es WHERE sourceItemID=I.itemID) AS numNotes, (SELECT COUNT(*) FROM itemAttachmen
ts WHERE sourceItemID=I.itemID) AS numAttachments FROM items I WHERE 1

zotero(3): Releasing session lock

zotero(3): HTTP POST version=2&sessionid=cc0636754793a48d1b7b18a1c31a850e to htt
ps://sync.zotero.org/unlock

zotero(1): [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_
FAILURE) [mozIStorageStatement.execute]" nsresult: "0x80004005 (NS_ERROR_FAILUR
E)" location: "JS frame :: chrome://zotero/content/xpcom/db.js :: anonymous ::
line 123" data: no] [QUERY: INSERT INTO tags (tagID, name, type, dateModified,
key) VALUES (?, ?, ?, ?, ?)] [ERROR: constraint failed]

zotero(3): <?xml version="1.0"?>
<response version="2" timestamp="1220892483"><unlocked/></response>

Had to chuckle at where it choked on my research.