App Store - file inconsistency: read 'q', expecting 'i'

Edit: Please read post 4!

I don't know what's wrong here.

2 week old MBA. App Store was really sluggish at first (constant beachball) but would download or update Apps after a little while.

Now, I cannot download or update Apps at all. I've tried disabling certificates as some posters have suggested, to no avail; I've also tried deleting the App Store cache, again to no avail. Any suggestions?

For reference, this is what I see when I try to download/update:



Just a constant ticker in the top left, which stays there until I go to another page or close the store.

Right, so I've tried a few more things, and I've worked out what the problem is.

Well, kinda. I've gone into Time Machine, and gone to my oldest backup (29th July), and taken the App Store, dragged it to my desktop. It works!

However, I can't restore, or replace the version of App Store in my Apps folder / launchpad, I'm presented with the following error:



"App Store.app can't be modified or deleted because it's required by Mac OSX"

How can I make it restore the App Store?

Ok, I've followed some advice, and when I launch through Terminal, I get the following error:

2011-08-17 22:36:54.238 App Store[1101:207] ### Error trying to unarchive data. *** file inconsistency: read 'q', expecting 'i'
2011-08-17 22:37:29.042 App Store[1101:207] *** file inconsistency: read 'q', expecting 'i'
2011-08-17 22:37:29.043 App Store[1101:207] (
0 CoreFoundation 0x9722fe77 __raiseError + 231
1 libobjc.A.dylib 0x94de4149 objc_exception_throw + 155
2 CoreFoundation 0x97197e89 +[NSException raise:format:arguments:] + 137
3 CoreFoundation 0x97197df9 +[NSException raise:format:] + 57
4 Foundation 0x99c374f8 checkExpected + 294
5 Foundation 0x99c38004 -[NSUnarchiver decodeValueOfObjCType:at:] + 55
6 CommerceKit 0x0014d377 -[ISAccount initWithCoder:] + 635
7 Foundation 0x99c37635 _decodeObject_old + 309
8 Foundation 0x99c384cd _decodeValueOfObjCType + 1125
9 Foundation 0x99c38013 -[NSUnarchiver decodeValueOfObjCType:at:] + 70
10 Foundation 0x99c37ca8 -[NSArray(NSArray) initWithCoder:] + 382
11 Foundation 0x99c37635 _decodeObject_old + 309
12 Foundation 0x99c369fd +[NSUnarchiver unarchiveObjectWithData:] + 89
13 CommerceKit 0x00122963 SSGetUnarchivedValueForKey + 122
14 CommerceKit 0x0014f99d -[ISAccountStore _accountsChangedNotification:] + 56
15 Foundation 0x99c3e470 __-[NSDistributedNotificationCenter addObserver:selector:namebject:suspensionBehavior:]_block_invoke_1 + 49
16 CoreFoundation 0x9717a9c0 ____CFXNotificationPostToken_block_invoke_1 + 352
17 CoreFoundation 0x97148830 __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 16
18 CoreFoundation 0x970fbcb1 __CFRunLoopDoBlocks + 337
19 CoreFoundation 0x971269c8 __CFRunLoopRun + 1096
20 CoreFoundation 0x971261ec CFRunLoopRunSpecific + 332
21 CoreFoundation 0x97126098 CFRunLoopRunInMode + 120
22 HIToolbox 0x98683487 RunCurrentEventLoopInMode + 318
23 HIToolbox 0x9868acee ReceiveNextEventCommon + 168
24 HIToolbox 0x9868ac32 BlockUntilNextEventMatchingListInMode + 88
25 AppKit 0x9b8e36c0 _DPSNextEvent + 678
26 AppKit 0x9b8e2f2d -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 113
27 AppKit 0x9b8df29f -[NSApplication run] + 904
28 AppKit 0x9bb72928 NSApplicationMain + 1054
29 App Store 0x000d15d1 App Store + 5585

Any advice on what this means and how I fix it?

Would really welcome any input anyone can give me on this, I have no clue what I'm doing.