Boot Failures
No boot device / missing OS: POST passes but OS fails to load. Check: drive connected, boot order in BIOS (correct drive first), drive detected in BIOS. MBR corruption: 'Missing operating system' or 'No boot device found.' Fix: boot from Windows installation media → Repair your computer → Troubleshoot → Advanced Options → Command Prompt. Run: `bootrec /fixmbr`, `bootrec /fixboot`, `bootrec /rebuildbcd`. BCD (Boot Configuration Data) corruption: 'Bootmgr is missing' error. Fix: `bootrec /rebuildbcd` or `bcdedit` manually. NTLDR is missing: very old Windows XP error — boot from XP CD → Recovery Console → `fixmbr`, `fixboot`. Windows failed to start: Safe Mode recommended: hold F8 (older Windows) or Shift+F8, or use recovery environment. Safe Mode troubleshooting: if Windows boots in Safe Mode but not normal mode → startup program or driver is causing the issue.
Blue Screen of Death (BSOD)
BSOD (Stop error): indicates a fatal system error in the Windows kernel or a driver. BSOD information: Stop code (e.g., IRQL_NOT_LESS_OR_EQUAL, PAGE_FAULT_IN_NONPAGED_AREA), faulting module (often a driver filename). Common stop codes: 0x0000007B (INACCESSIBLE_BOOT_DEVICE) — boot drive not accessible, driver issue. 0x0000000A (IRQL_NOT_LESS_OR_EQUAL) — driver memory access violation. 0x0000001E (KMODE_EXCEPTION_NOT_HANDLED) — driver exception. 0x000000EF (CRITICAL_PROCESS_DIED) — essential process crashed. BSOD troubleshooting: note the stop code and faulting module. Check minidumps in C:\Windows\Minidump with WinDbg or BlueScreenView. If a recent driver install caused it: roll back driver (Device Manager → Driver tab → Roll Back Driver). If random: run Windows Memory Diagnostic (RAM issue is common BSOD cause). If after Windows Update: uninstall the recent update.
Slow Performance
Slow Windows performance diagnostic approach: Task Manager → check CPU, RAM, Disk, Network for resource bottlenecks. CPU: consistently >80% → identify the process, check for malware, check startup items. RAM: if 'In Use' RAM is close to total installed RAM → insufficient RAM, or memory leak. Disk: consistently near 100% active time → HDD is a bottleneck, or Windows Search/Superfetch causing excessive I/O (disable on HDDs). Network: check for excessive background downloads. Common causes: too many startup programs (disable in Task Manager → Startup). Insufficient RAM (especially for Windows 10/11 — 8 GB minimum, 16 GB recommended). Aging HDD — upgrade to SSD dramatically improves performance. Malware consuming resources. Fragmented HDD (SSDs don't need defragmentation — Windows automatically runs defragmentation on HDDs, TRIM on SSDs). Low disk space (<15% free — Windows needs space for temp files and pagefile). Windows performance tools: Task Manager, Resource Monitor, Performance Monitor (perfmon.exe).
Startup Problems
Startup repair: Windows Recovery Environment (WinRE) → Troubleshoot → Advanced Options → Startup Repair (automatically diagnoses and fixes boot issues). System Restore: roll back to a previous working state (does not affect personal files). msconfig: System Configuration → General tab → Selective Startup to isolate startup problems. Disable all non-Microsoft services, disable all startup items → restart → if fixed, re-enable in halves to isolate. Services tab: Hide all Microsoft services → Disable all → restart. If fixed, a third-party service was the culprit. Driver issues: a recently installed driver causing boot failure — boot to Safe Mode (no third-party drivers loaded) → Device Manager → roll back or uninstall the driver. Windows 10/11 Advanced Startup options (hold Shift + Restart): Startup Settings (access Safe Mode options), Startup Repair, System Restore, Command Prompt, System Image Recovery.
Corrupted System Files
Windows system file corruption can cause crashes, missing features, and error messages. SFC (System File Checker): `sfc /scannow` (admin command prompt). Scans all protected Windows files. Replaces corrupted files from a cached copy. If SFC reports it couldn't fix all files → run DISM first. DISM (Deployment Image Servicing and Management): `DISM /Online /Cleanup-Image /RestoreHealth`. Downloads a clean Windows image from Microsoft to use for repairs. Run this before SFC if SFC fails. Verify: `sfc /verifyonly` (report only, no fix). SFC log: `%windir%\Logs\CBS\CBS.log` — view detailed SFC scan results. Windows RE reinstall: if file corruption is severe, 'Reset this PC → Keep my files' reinstalls Windows while preserving personal documents. Full reinstall: wipe and reinstall from installation media for the cleanest result.