Frequent blue screen crash dumps

Hello, my Dell Inspiron laptop (Windows 7) has been crashing often these days. I am not tech savvy enough to understand the problem. I googled and download BlueScreenView software to get the dump files. I cannot understand the meaning of these and need help to resolve the crash dumps issue. Here are the dump file details from the last 4 dump files.

Code:

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Dump File : 090412-26738-01.dmp
Crash Time : 9/4/12 10:04:31 AM
Bug Check String : APC_INDEX_MISMATCH
Bug Check Code : 0x00000001
Parameter 1 : 00000000`750e2e09
Parameter 2 : 00000000`00000000
Parameter 3 : 00000000`0000ffff
Parameter 4 : fffff880`0758eca0
Caused By Driver : ntoskrnl.exe
Caused By Address : ntoskrnl.exe+7f1c0
File Description : NT Kernel & System
Product Name : Microsoft® Windows® Operating System
Company : Microsoft Corporation
File Version : 6.1.7601.17835 (win7sp1_gdr.120503-2030)
Processor : x64
Crash Address : ntoskrnl.exe+7f1c0
Stack Address 1 : 
Stack Address 2 : 
Stack Address 3 : 
Computer Name : 
Full Path : C:\Windows\Minidump\090412-26738-01.dmp
Processors Count : 4
Major Version : 15
Minor Version : 7601
Dump File Size : 275,632
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Dump File : 082912-33228-01.dmp
Crash Time : 8/29/12 11:28:22 AM
Bug Check String : 
Bug Check Code : 0x00000116
Parameter 1 : fffffa80`088eb4e0
Parameter 2 : fffff880`01006c48
Parameter 3 : 00000000`00000000
Parameter 4 : 00000000`00000002
Caused By Driver : dxgkrnl.sys
Caused By Address : dxgkrnl.sys+5d000
File Description : 
Product Name : 
Company : 
File Version : 
Processor : x64
Crash Address : ntoskrnl.exe+7f1c0
Stack Address 1 : 
Stack Address 2 : 
Stack Address 3 : 
Computer Name : 
Full Path : C:\Windows\Minidump\082912-33228-01.dmp
Processors Count : 4
Major Version : 15
Minor Version : 7601
Dump File Size : 372,952
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Dump File : 082312-35708-01.dmp
Crash Time : 8/23/12 1:42:26 PM
Bug Check String : 
Bug Check Code : 0x00000116
Parameter 1 : fffffa80`0cea34e0
Parameter 2 : fffff880`04ae4c48
Parameter 3 : 00000000`00000000
Parameter 4 : 00000000`00000002
Caused By Driver : dxgkrnl.sys
Caused By Address : dxgkrnl.sys+5d000
File Description : 
Product Name : 
Company : 
File Version : 
Processor : x64
Crash Address : ntoskrnl.exe+7f1c0
Stack Address 1 : 
Stack Address 2 : 
Stack Address 3 : 
Computer Name : 
Full Path : C:\Windows\Minidump\082312-35708-01.dmp
Processors Count : 4
Major Version : 15
Minor Version : 7601
Dump File Size : 372,960
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Dump File : 080612-21309-01.dmp
Crash Time : 8/6/12 11:32:28 AM
Bug Check String : APC_INDEX_MISMATCH
Bug Check Code : 0x00000001
Parameter 1 : 00000000`745b2e09
Parameter 2 : 00000000`00000000
Parameter 3 : 00000000`0000ffff
Parameter 4 : fffff880`06e3aca0
Caused By Driver : ntoskrnl.exe
Caused By Address : ntoskrnl.exe+7f1c0
File Description : NT Kernel & System
Product Name : Microsoft® Windows® Operating System
Company : Microsoft Corporation
File Version : 6.1.7601.17835 (win7sp1_gdr.120503-2030)
Processor : x64
Crash Address : ntoskrnl.exe+7f1c0
Stack Address 1 : 
Stack Address 2 : 
Stack Address 3 : 
Computer Name : 
Full Path : C:\Windows\Minidump\080612-21309-01.dmp
Processors Count : 4
Major Version : 15
Minor Version : 7601
Dump File Size : 275,632
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The other dump files also show Caused by mostly ntoskrnl.exe or dxgkrnl.sys. Please suggest what may be causing the problem and how to resolve it. Thanks!