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2. Virus warnings popping up from an antivirus that you didn't intentionally install (the antivirus program is a fake and tries to claim you have scary sounding viruses with names like 'bankpasswordstealer.vir'. You're encouraged to pay for this program to clean these).
3. Web pages redirected or blocked, for example, home pages of AV products or support sites (www.symantec.com, www.avg.com, www.microsoft.com) are redirected to sites filled with adverts, or fake sites promoting bogus anti virus / "helpful" removal tools, or are blocked altogether.
Increased startup time
4. Random failures and things happening when they shouldn't (e.g. some new viruses put group policy restrictions on your machine to prevent task manager or other diagnostic programs from running).