With regards to the other sciences, you shouldn't be suffering from the "Daily quota of 1 tasks" issue - thus, could you please post the first 30 odd lines of your BOINC messages tab (so as we can see your set-up), along with all the errors you're seeing around these messages you're reporting.

Also, if you could go into your Results page and paste any/all error messages you're seeing (over and above any you've already reported for HCC - that's a special case, as it's possibly GPU related).

Installing Catalyst now.

Here's the dump of my event log (first 30ish lines).

10/9/2012 6:40:47 AM | | No config file found - using defaults
10/9/2012 6:40:47 AM | | Starting BOINC client version 7.0.28 for windows_intelx86
10/9/2012 6:40:47 AM | | log flags: file_xfer, sched_ops, task
10/9/2012 6:40:47 AM | | Libraries: libcurl/7.25.0 OpenSSL/1.0.1 zlib/1.2.6
10/9/2012 6:40:47 AM | | Data directory: E:\Program Files\BOINC\Data
10/9/2012 6:40:47 AM | | Running under account user
10/9/2012 6:40:47 AM | | Processor: 4 GenuineIntel Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz [Family 6 Model 15 Stepping 11]
10/9/2012 6:40:47 AM | | Processor: 4.00 MB cache
10/9/2012 6:40:47 AM | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 cx16 nx lm vmx tm2 pbe
10/9/2012 6:40:47 AM | | OS: Microsoft Windows XP: Professional x86 Edition, Service Pack 3, (05.01.2600.00)
10/9/2012 6:40:47 AM | | Memory: 3.25 GB physical, 5.09 GB virtual
10/9/2012 6:40:47 AM | | Disk: 390.63 GB total, 82.92 GB free
10/9/2012 6:40:47 AM | | Local time is UTC -4 hours
10/9/2012 6:40:47 AM | | ATI GPU 0: ATI Radeon HD 5700 series (Juniper) (CAL version 1.4.1417, 1024MB, 999MB available, 2720 GFLOPS peak)
10/9/2012 6:40:47 AM | | [error] bad value 1350095458.593750 of time stats last update; ignoring
10/9/2012 6:40:47 AM | World Community Grid | URL http://www.worldcommunitygrid.org/; Computer ID 1451328; resource share 100
10/9/2012 6:40:47 AM | | General prefs: from http://bam.boincstats.com/ (last modified 11-Apr-2012 06:25:05)
10/9/2012 6:40:47 AM | | Host location: none
10/9/2012 6:40:47 AM | | General prefs: using your defaults
10/9/2012 6:40:47 AM | | Reading preferences override file
10/9/2012 6:40:47 AM | | Preferences:
10/9/2012 6:40:47 AM | | max memory usage when active: 1663.05MB
10/9/2012 6:40:47 AM | | max memory usage when idle: 2993.49MB
10/9/2012 6:40:47 AM | | max disk usage: 33.83GB
10/9/2012 6:40:47 AM | | don't use GPU while active
10/9/2012 6:40:47 AM | | (to change preferences, visit the web site of an attached project, or select Preferences in the Manager)
10/9/2012 6:40:47 AM | | Not using a proxy
10/9/2012 6:40:48 AM | World Community Grid | File 7e0ae940e05e229e4f6e40b40451dded.zip exists already, skipping download
10/9/2012 6:40:48 AM | World Community Grid | File dsfl.target_00020-31.pdbqt exists already, skipping download
10/9/2012 6:40:48 AM | World Community Grid | Started download of DSFL_00020-31_0000005_0384_DSFL_00020-31_0000005_0384.job
10/9/2012 6:40:48 AM | World Community Grid | Started download of DSFL_00020-31_0000005_0384_DSFL_00020-31_0000005_0384.zip
10/9/2012 6:40:48 AM | World Community Grid | Restarting task HFCC_target-10_00394520_target-10_0001_0 using hfcc version 640 in slot 3
10/9/2012 6:40:48 AM | World Community Grid | Restarting task HFCC_target-10_00394484_target-10_0000_0 using hfcc version 640 in slot 0
10/9/2012 6:40:48 AM | World Community Grid | Restarting task HFCC_target-10_00416036_target-10_0001_0 using hfcc version 640 in slot 2
10/9/2012 6:40:48 AM | World Community Grid | Restarting task DSFL_00020-30_0000008_0487_0 using dsfl version 625 in slot 1
10/9/2012 6:40:48 AM | World Community Grid | Sending scheduler request: To fetch work.
10/9/2012 6:40:48 AM | World Community Grid | Requesting new tasks for ATI
10/9/2012 6:40:55 AM | World Community Grid | Finished download of DSFL_00020-31_0000005_0384_DSFL_00020-31_0000005_0384.job

Mine is a 67xx and Boinc also reports it as a 5700 series but it's working anyway.

The original OP notes this error

- exit code -1073741515 (0xc0000135)

The first and the 3rd reply give the answer to fixing that [installing the ATI-SDK].

Think that XP is not far off from getting dropped support. May have seen a comment already in the developers mail list to that effect, meaning future clients wont be installing/running properly on this age-old OS, superseded by Vista > Windows 7 > Windows 8 [end of this week, Friday October 26]. Myself, been running Linux on my Q6600 since a few years. Does the DSFL/SN2S/GFAM 60-65% faster than Windows 7 on the same machine. Maybe worth considering once the HCC-GPU is starting to be beta tested or goes to production [if we ever get that far]